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/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])