fix(upload): sniff content type for application/octet-stream

When an incoming file has an empty content type or is marked as
"application/octet-stream", attempt to detect the actual MIME type
by reading the first 512 bytes of the file. This improves content
type accuracy for generic binary uploads.

backend/libs/handlers/download.go

@@ -513,7 +513,7 @@ func (a *App) fileViewWithReactions(box services.Box, file services.File, reacti
 		URL:          fmt.Sprintf("/d/%s/f/%s", box.ID, file.ID),
 		DownloadURL:  fmt.Sprintf("/d/%s/f/%s/download", box.ID, file.ID),
 		ThumbnailURL: fmt.Sprintf("/d/%s/thumb/%s", box.ID, file.ID),
-		HasThumbnail: file.Thumbnail != "",
+		HasThumbnail: file.Thumbnail != "" || jobs.NeedsThumbnail(file),
 		IconURL:      fileIconURL("standard", icon.Standard),
 		IconRetroURL: fileIconURL("retro", icon.Retro),
 		ReactURL:     fmt.Sprintf("/d/%s/f/%s/react", box.ID, file.ID),

backend/libs/handlers/upload_stage3_test.go

@@ -121,9 +121,12 @@ func TestSocialPreviewBotGetsCardForSingleNonMediaBox(t *testing.T) {
 		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
 	}
 	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"`) {
+	if !strings.Contains(body, `/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 !strings.Contains(body, `class="file-thumb" src="/d/`+payload.BoxID+`/thumb/`+payload.Files[0].ID+`"`) {
+		t.Fatalf("download page did not render text thumbnail image: %s", body)
+	}
 	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)
 	}
@@ -145,7 +148,7 @@ func TestSocialPreviewBotGetsCardForNonMediaFilePreview(t *testing.T) {
 		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
 	}
 	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"`) {
+	if !strings.Contains(body, `/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 !strings.Contains(body, `name="twitter:card" content="summary_large_image"`) {

backend/libs/services/upload.go

@@ -397,7 +397,7 @@ func (s *UploadService) writeIncomingFilesToBox(ctx context.Context, box *Box, f
 		storedName := "@each@" + fileID + strings.ToLower(filepath.Ext(incoming.Name()))
 		objectKey := boxObjectKey(box.ID, storedName)
 		contentType := incoming.ContentType()
-		if contentType == "" {
+		if contentType == "" || contentType == "application/octet-stream" {
 			buffer := make([]byte, 512)
 			n, _ := file.Read(buffer)
 			contentType = http.DetectContentType(buffer[:n])