feat(backend): enhance social previews for single-file shares

Implements dynamic Open Graph (OG) metadata and image generation for
single-file shared boxes to improve social media previews.

Changes include:
- Added a new route `/d/{boxID}/f/{fileID}/og-image.jpg` for file-specific OG images.
- Updated `DownloadPage` to dynamically set the page title, description, and OG image properties when a box contains only one file.
- Restricted raw media inline serving for social bots to images and videos.
- Added helper functions to format file share descriptions and determine appropriate social image URLs and types.
- Integrated basic font rendering to support dynamic OG image generation.

backend/libs/handlers/app.go

@@ -132,6 +132,7 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("POST /d/{boxID}/f/{fileID}/react", a.ReactToFile)
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}", a.DownloadFile)
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}/download", a.DownloadFileContent)
+	mux.HandleFunc("GET /d/{boxID}/f/{fileID}/og-image.jpg", a.FileOGImage)
 	mux.HandleFunc("GET /d/{boxID}/thumb/{fileID}", a.Thumbnail)
 	mux.HandleFunc("GET /d/{boxID}/og-image.jpg", a.BoxOGImage)
 	mux.HandleFunc("GET /robots.txt", a.RobotsTxt)

backend/libs/handlers/download.go

@@ -104,13 +104,16 @@ func (a *App) DownloadPage(w http.ResponseWriter, r *http.Request) {
 	}
 	locked := a.uploadService.IsProtected(box) && !a.isBoxUnlocked(r, box)
 	if isSocialPreviewBot(r) && !locked && len(box.Files) == 1 {
-		if box.Files[0].Processing {
+		file := box.Files[0]
+		if file.Processing {
 			http.Error(w, "file is still processing", http.StatusAccepted)
 			return
 		}
-		a.serveFileContent(w, r, box, box.Files[0], false)
-		a.logger.Info("single-file box served inline for social preview", withRequestLogAttrs(r, "source", "download", "severity", "user_activity", "code", 2008, "box_id", box.ID, "file_id", box.Files[0].ID)...)
-		return
+		if shouldServeRawSocialMedia(file) {
+			a.serveFileContent(w, r, box, file, false)
+			a.logger.Info("single-file media served inline for social preview", withRequestLogAttrs(r, "source", "download", "severity", "user_activity", "code", 2008, "box_id", box.ID, "file_id", file.ID)...)
+			return
+		}
 	}
 	visitorID := a.reactionVisitorID(w, r)
 	reactionsByFile, reactedByFile, err := a.reactionService.SummaryForBox(box.ID, visitorID)
@@ -132,13 +135,25 @@ func (a *App) DownloadPage(w http.ResponseWriter, r *http.Request) {
 	expiresLabel := boxExpiryLabel(box.ExpiresAt, "Jan 2, 2006 15:04 MST")
 	title := "Shared files on Warpbox"
 	description := fmt.Sprintf("%d file%s shared via Warpbox · expires %s", len(box.Files), plural(len(box.Files)), expiresLabel)
+	ogImage := absoluteURL(r, fmt.Sprintf("/d/%s/og-image.jpg", box.ID))
+	imageAlt := fmt.Sprintf("%d shared file%s on Warp Box", len(box.Files), plural(len(box.Files)))
+	imageType := "image/jpeg"
+	if !locked && len(box.Files) == 1 && !box.Files[0].Processing {
+		file := box.Files[0]
+		view := a.fileView(box, file)
+		fileSize := helpers.FormatBytes(file.Size)
+		title = file.Name
+		description = fileShareDescription(fileSize, file.ContentType, box.ExpiresAt)
+		ogImage = socialImageURL(r, box, file, view)
+		imageAlt = fmt.Sprintf("Download card for %s", file.Name)
+		imageType = socialImageType(file)
+	}
 	if locked && box.Obfuscate {
 		title = "Protected Warpbox link"
 		description = "This shared box is password protected."
 	}
 
 	pageURL := absoluteURL(r, fmt.Sprintf("/d/%s", box.ID))
-	ogImage := absoluteURL(r, fmt.Sprintf("/d/%s/og-image.jpg", box.ID))
 
 	// All user uploads are private/temporary — noindex by default.
 	robots := web.RobotsNone
@@ -149,7 +164,8 @@ func (a *App) DownloadPage(w http.ResponseWriter, r *http.Request) {
 		CanonicalURL: pageURL,
 		Robots:       robots,
 		ImageURL:     ogImage,
-		ImageAlt:     fmt.Sprintf("%d shared file%s on Warp Box", len(box.Files), plural(len(box.Files))),
+		ImageAlt:     imageAlt,
+		ImageType:    imageType,
 		Data: downloadPageData{
 			Box:           boxView{ID: box.ID},
 			Files:         files,
@@ -172,6 +188,43 @@ func plural(n int) string {
 	return "s"
 }
 
+func shouldServeRawSocialMedia(file services.File) bool {
+	return file.PreviewKind == "image" || file.PreviewKind == "video"
+}
+
+func fileShareDescription(size, contentType string, expiresAt time.Time) string {
+	if strings.TrimSpace(contentType) == "" {
+		contentType = "file"
+	}
+	return fmt.Sprintf("%s · %s · click to preview or download · expires %s", size, contentType, boxExpiryLabel(expiresAt, "Jan 2, 2006"))
+}
+
+func socialImageURL(r *http.Request, box services.Box, file services.File, view fileView) string {
+	if file.PreviewKind == "image" {
+		return absoluteURL(r, view.DownloadURL+"?inline=1")
+	}
+	if file.PreviewKind == "video" && view.HasThumbnail {
+		return absoluteURL(r, view.ThumbnailURL)
+	}
+	return absoluteURL(r, fmt.Sprintf("/d/%s/f/%s/og-image.jpg", box.ID, file.ID))
+}
+
+func socialImageType(file services.File) string {
+	if file.PreviewKind == "image" {
+		return file.ContentType
+	}
+	return "image/jpeg"
+}
+
+func socialOGType(file services.File) string {
+	switch file.PreviewKind {
+	case "video":
+		return "video.other"
+	default:
+		return "website"
+	}
+}
+
 func (a *App) DownloadFile(w http.ResponseWriter, r *http.Request) {
 	box, file, ok := a.loadFileForRequest(w, r)
 	if !ok {
@@ -184,21 +237,30 @@ func (a *App) DownloadFile(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "file is still processing", http.StatusAccepted)
 			return
 		}
-		a.serveFileContent(w, r, box, file, false)
-		a.logger.Info("file served inline for social preview", withRequestLogAttrs(r, "source", "download", "severity", "user_activity", "code", 2009, "box_id", box.ID, "file_id", file.ID)...)
-		return
+		if shouldServeRawSocialMedia(file) {
+			a.serveFileContent(w, r, box, file, false)
+			a.logger.Info("media file served inline for social preview", withRequestLogAttrs(r, "source", "download", "severity", "user_activity", "code", 2009, "box_id", box.ID, "file_id", file.ID)...)
+			return
+		}
 	}
 	view := a.fileView(box, file)
 	fileSize := helpers.FormatBytes(file.Size)
 	title := file.Name
-	description := fmt.Sprintf("%s · %s file shared via Warp Box", fileSize, file.ContentType)
-	imageURL := absoluteURL(r, view.ThumbnailURL)
-	imageAlt := fmt.Sprintf("Preview of %s", file.Name)
+	description := fileShareDescription(fileSize, file.ContentType, box.ExpiresAt)
+	imageURL := socialImageURL(r, box, file, view)
+	imageAlt := fmt.Sprintf("Download card for %s", file.Name)
+	ogType := socialOGType(file)
+	mediaURL := ""
+	if file.PreviewKind == "video" {
+		mediaURL = absoluteURL(r, view.DownloadURL+"?inline=1")
+	}
 	if locked && box.Obfuscate {
 		title = "Protected Warpbox file"
 		description = "This shared file is password protected."
 		imageURL = absoluteURL(r, "/static/img/file-placeholder.webp")
 		imageAlt = "Password protected file on Warp Box"
+		ogType = "website"
+		mediaURL = ""
 	}
 
 	pageURL := absoluteURL(r, fmt.Sprintf("/d/%s/f/%s", box.ID, file.ID))
@@ -208,8 +270,12 @@ func (a *App) DownloadFile(w http.ResponseWriter, r *http.Request) {
 		Description:  description,
 		CanonicalURL: pageURL,
 		Robots:       web.RobotsNone,
+		OGType:       ogType,
 		ImageURL:     imageURL,
 		ImageAlt:     imageAlt,
+		ImageType:    socialImageType(file),
+		MediaURL:     mediaURL,
+		MediaType:    file.ContentType,
 		Data: previewPageData{
 			Box:         boxView{ID: box.ID},
 			File:        view,

backend/libs/handlers/ogimage.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"bytes"
+	"fmt"
 	"image"
 	"image/color"
 	"image/draw"
@@ -11,10 +12,18 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"strings"
 	"time"
 
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/basicfont"
+	"golang.org/x/image/font/opentype"
+	"golang.org/x/image/math/fixed"
 	xdraw "golang.org/x/image/draw"
 	_ "golang.org/x/image/webp"
+
+	"warpbox.dev/backend/libs/helpers"
+	"warpbox.dev/backend/libs/services"
 )
 
 // Open Graph image dimensions recommended for large summary cards
@@ -74,6 +83,22 @@ func (a *App) BoxOGImage(w http.ResponseWriter, r *http.Request) {
 	a.serveOGImage(w, r, renderCollage(thumbs))
 }
 
+// FileOGImage renders a branded card for files that should not be served as raw
+// media to social preview bots: text, Markdown, HTML, PDF, audio, archives, etc.
+func (a *App) FileOGImage(w http.ResponseWriter, r *http.Request) {
+	box, file, ok := a.loadFileForRequest(w, r)
+	if !ok {
+		return
+	}
+	if a.uploadService.IsProtected(box) && box.Obfuscate && !a.isBoxUnlocked(r, box) {
+		a.serveOGImage(w, r, a.ogPlaceholder())
+		return
+	}
+
+	icon := a.ogFileIcon(file)
+	a.serveOGImage(w, r, a.renderFileCard(file, icon))
+}
+
 func (a *App) serveOGImage(w http.ResponseWriter, r *http.Request, img image.Image) {
 	var buf bytes.Buffer
 	if err := jpeg.Encode(&buf, img, &jpeg.Options{Quality: 85}); err != nil {
@@ -115,6 +140,158 @@ func (a *App) ogPlaceholder() image.Image {
 	return canvas
 }
 
+func (a *App) ogFileIcon(file services.File) image.Image {
+	if a.fileIcons == nil {
+		return nil
+	}
+	icon := a.fileIcons.lookup(file.Name, file.ContentType)
+	if icon.Retro == "" {
+		return nil
+	}
+	path := filepath.Join(a.cfg.StaticDir, "file-icons", "retro", icon.Retro)
+	f, err := os.Open(path)
+	if err != nil {
+		return nil
+	}
+	defer f.Close()
+	img, _, err := image.Decode(f)
+	if err != nil {
+		return nil
+	}
+	return img
+}
+
+func (a *App) renderFileCard(file services.File, icon image.Image) image.Image {
+	canvas := image.NewRGBA(image.Rect(0, 0, ogImageWidth, ogImageHeight))
+	draw.Draw(canvas, canvas.Bounds(), &image.Uniform{ogBackground}, image.Point{}, draw.Src)
+
+	panel := image.Rect(70, 72, ogImageWidth-70, ogImageHeight-72)
+	draw.Draw(canvas, panel, &image.Uniform{color.RGBA{R: 0x17, G: 0x14, B: 0x2d, A: 0xff}}, image.Point{}, draw.Src)
+	draw.Draw(canvas, image.Rect(panel.Min.X, panel.Min.Y, panel.Max.X, panel.Min.Y+6), &image.Uniform{color.RGBA{R: 0xa7, G: 0x8b, B: 0xfa, A: 0xff}}, image.Point{}, draw.Src)
+
+	titleFace := a.ogFont(44, true)
+	bodyFace := a.ogFont(28, false)
+	metaFace := a.ogFont(24, false)
+	buttonFace := a.ogFont(26, true)
+
+	if icon != nil {
+		xdraw.NearestNeighbor.Scale(canvas, image.Rect(110, 142, 230, 262), icon, icon.Bounds(), xdraw.Over, nil)
+	} else {
+		draw.Draw(canvas, image.Rect(110, 142, 230, 262), &image.Uniform{color.RGBA{R: 0x21, G: 0x1b, B: 0x3e, A: 0xff}}, image.Point{}, draw.Src)
+	}
+
+	titleLines := wrapOGText(file.Name, titleFace, 850)
+	if len(titleLines) > 2 {
+		titleLines = titleLines[:2]
+		titleLines[1] = trimOGText(titleLines[1], titleFace, 850)
+	}
+	y := 156
+	for _, line := range titleLines {
+		drawOGText(canvas, titleFace, line, 265, y, color.RGBA{R: 0xf5, G: 0xf3, B: 0xff, A: 0xff})
+		y += 52
+	}
+
+	size := helpers.FormatBytes(file.Size)
+	typeLabel := strings.TrimSpace(file.ContentType)
+	if typeLabel == "" {
+		typeLabel = "application/octet-stream"
+	}
+	drawOGText(canvas, bodyFace, fmt.Sprintf("%s · %s", size, typeLabel), 265, y+12, color.RGBA{R: 0xaa, G: 0xa4, B: 0xd6, A: 0xff})
+
+	info := fileCardInfo(file)
+	for i, line := range wrapOGText(info, metaFace, 900) {
+		if i >= 2 {
+			break
+		}
+		drawOGText(canvas, metaFace, line, 110, 355+i*34, color.RGBA{R: 0xd8, G: 0xd2, B: 0xff, A: 0xff})
+	}
+
+	button := image.Rect(110, 474, 430, 548)
+	draw.Draw(canvas, button, &image.Uniform{color.RGBA{R: 0x8b, G: 0x5c, B: 0xf6, A: 0xff}}, image.Point{}, draw.Src)
+	drawOGText(canvas, buttonFace, "Click to download", 142, 520, color.RGBA{R: 0xff, G: 0xff, B: 0xff, A: 0xff})
+	drawOGText(canvas, metaFace, "warpbox.dev", 910, 520, color.RGBA{R: 0xaa, G: 0xa4, B: 0xd6, A: 0xff})
+	return canvas
+}
+
+func fileCardInfo(file services.File) string {
+	switch {
+	case strings.HasPrefix(file.ContentType, "audio/"):
+		return "Audio file shared through Warpbox. Open the link to preview in your browser or download the original."
+	case file.ContentType == "text/markdown":
+		return "Markdown file shared through Warpbox. Open the link to view the rendered preview, source, or download."
+	case strings.Contains(file.ContentType, "html"):
+		return "HTML file shared through Warpbox. Open the link to preview rendered HTML, source, or download."
+	case strings.Contains(file.ContentType, "pdf"):
+		return "PDF file shared through Warpbox. Open the link to download the original file."
+	case strings.HasPrefix(file.ContentType, "text/"):
+		return "Text file shared through Warpbox. Open the link to preview the content or download."
+	default:
+		return "File shared through Warpbox. Open the link to preview available details or download the original."
+	}
+}
+
+func (a *App) ogFont(size float64, bold bool) font.Face {
+	name := "PixeloidSans.ttf"
+	if bold {
+		name = "PixeloidSans-Bold.ttf"
+	}
+	data, err := os.ReadFile(filepath.Join(a.cfg.StaticDir, "fonts", "pixeloid_sans", name))
+	if err != nil {
+		return basicfont.Face7x13
+	}
+	parsed, err := opentype.Parse(data)
+	if err != nil {
+		return basicfont.Face7x13
+	}
+	face, err := opentype.NewFace(parsed, &opentype.FaceOptions{Size: size, DPI: 72, Hinting: font.HintingFull})
+	if err != nil {
+		return basicfont.Face7x13
+	}
+	return face
+}
+
+func drawOGText(dst *image.RGBA, face font.Face, text string, x, y int, c color.Color) {
+	d := font.Drawer{
+		Dst:  dst,
+		Src:  image.NewUniform(c),
+		Face: face,
+		Dot:  fixed.P(x, y),
+	}
+	d.DrawString(text)
+}
+
+func wrapOGText(text string, face font.Face, maxWidth int) []string {
+	words := strings.Fields(text)
+	if len(words) == 0 {
+		return []string{text}
+	}
+	lines := []string{}
+	current := words[0]
+	for _, word := range words[1:] {
+		next := current + " " + word
+		if ogTextWidth(face, next) <= maxWidth {
+			current = next
+			continue
+		}
+		lines = append(lines, current)
+		current = word
+	}
+	lines = append(lines, current)
+	return lines
+}
+
+func trimOGText(text string, face font.Face, maxWidth int) string {
+	for ogTextWidth(face, text+"...") > maxWidth && len(text) > 1 {
+		text = text[:len(text)-1]
+	}
+	return strings.TrimSpace(text) + "..."
+}
+
+func ogTextWidth(face font.Face, text string) int {
+	bounds, _ := font.BoundString(face, text)
+	return (bounds.Max.X - bounds.Min.X).Ceil()
+}
+
 // renderCollage tiles up to four thumbnails into the OG canvas with a small gap.
 func renderCollage(thumbs []image.Image) image.Image {
 	canvas := image.NewRGBA(image.Rect(0, 0, ogImageWidth, ogImageHeight))

backend/libs/handlers/upload_stage3_test.go

@@ -106,7 +106,7 @@ func TestUploadTextResponseReturnsOnlyBoxURL(t *testing.T) {
 	}
 }
 
-func TestSocialPreviewBotGetsRawSingleFileBox(t *testing.T) {
+func TestSocialPreviewBotGetsCardForSingleNonMediaBox(t *testing.T) {
 	app, cleanup := newTestApp(t)
 	defer cleanup()
 	payload := uploadThroughApp(t, app)
@@ -120,15 +120,16 @@ func TestSocialPreviewBotGetsRawSingleFileBox(t *testing.T) {
 	if response.Code != http.StatusOK {
 		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
 	}
-	if strings.Contains(response.Body.String(), "Shared files on Warpbox") {
-		t.Fatalf("social preview bot received HTML download page")
+	body := response.Body.String()
+	if !strings.Contains(body, `property="og:image" content="http://example.test/d/`+payload.BoxID+`/f/`+payload.Files[0].ID+`/og-image.jpg"`) {
+		t.Fatalf("social preview bot did not receive file card metadata: %s", body)
 	}
-	if response.Body.String() != "hello" {
-		t.Fatalf("social preview body = %q", response.Body.String())
+	if !strings.Contains(body, "Click to preview or download") && !strings.Contains(body, "click to preview or download") {
+		t.Fatalf("social preview body missing preview/download description: %s", body)
 	}
 }
 
-func TestSocialPreviewBotGetsRawFilePreview(t *testing.T) {
+func TestSocialPreviewBotGetsCardForNonMediaFilePreview(t *testing.T) {
 	app, cleanup := newTestApp(t)
 	defer cleanup()
 	payload := uploadThroughApp(t, app)
@@ -143,11 +144,46 @@ func TestSocialPreviewBotGetsRawFilePreview(t *testing.T) {
 	if response.Code != http.StatusOK {
 		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
 	}
-	if strings.Contains(response.Body.String(), "preview-title") {
-		t.Fatalf("social preview bot received HTML preview page")
+	body := response.Body.String()
+	if !strings.Contains(body, `property="og:image" content="http://example.test/d/`+payload.BoxID+`/f/`+payload.Files[0].ID+`/og-image.jpg"`) {
+		t.Fatalf("social preview bot did not receive file card metadata: %s", body)
 	}
-	if response.Body.String() != "hello" {
-		t.Fatalf("social preview body = %q", response.Body.String())
+	if !strings.Contains(body, `name="twitter:card" content="summary_large_image"`) {
+		t.Fatalf("social preview body missing twitter card metadata: %s", body)
+	}
+}
+
+func TestSocialPreviewBotGetsRawImageFilePreview(t *testing.T) {
+	app, cleanup := newTestApp(t)
+	defer cleanup()
+
+	request := multipartUploadRequest(t, "/api/v1/upload", "file", "image.png", "\x89PNG\r\n\x1a\nimage")
+	request.Header.Set("Accept", "application/json")
+	uploadResponse := httptest.NewRecorder()
+	app.Upload(uploadResponse, request)
+	if uploadResponse.Code != http.StatusCreated {
+		t.Fatalf("upload status = %d, body = %s", uploadResponse.Code, uploadResponse.Body.String())
+	}
+	var payload services.UploadResult
+	if err := json.Unmarshal(uploadResponse.Body.Bytes(), &payload); err != nil {
+		t.Fatalf("json.Unmarshal returned error: %v", err)
+	}
+
+	previewRequest := httptest.NewRequest(http.MethodGet, "/d/"+payload.BoxID+"/f/"+payload.Files[0].ID, nil)
+	previewRequest.Header.Set("User-Agent", "Discordbot/2.0")
+	previewRequest.SetPathValue("boxID", payload.BoxID)
+	previewRequest.SetPathValue("fileID", payload.Files[0].ID)
+	response := httptest.NewRecorder()
+	app.DownloadFile(response, previewRequest)
+
+	if response.Code != http.StatusOK {
+		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
+	}
+	if strings.Contains(response.Body.String(), "preview-title") {
+		t.Fatalf("image social preview bot received HTML preview page")
+	}
+	if !strings.HasPrefix(response.Body.String(), "\x89PNG\r\n\x1a\n") {
+		t.Fatalf("image social preview body = %q", response.Body.String())
 	}
 }