Preserve uploaded file extensions

server/media/media.go

@@ -69,6 +69,58 @@ type AllowedOrigin struct {
 
 var fileNamePattern = regexp.MustCompile(`^[-_A-Za-z0-9]+`)
 
+var uploadExtensionMimeTypes = map[string]string{
+	".aac":  "audio/aac",
+	".m4a":  "audio/x-m4a",
+	".doc":  "application/msword",
+	".docx": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+	".ppt":  "application/vnd.ms-powerpoint",
+	".pptx": "application/vnd.openxmlformats-officedocument.presentationml.presentation",
+	".xls":  "application/vnd.ms-excel",
+	".xlsx": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
+}
+
+// MimeTypeForUpload picks the MIME type to persist for an uploaded file. Some common formats are
+// containers and are misdetected by http.DetectContentType, for example m4a as video/mp4 and Office
+// documents as zip, so the original file extension wins for those cases.
+func MimeTypeForUpload(detected, clientContentType, originalName string) string {
+	ext := ExtensionByFilename(originalName)
+	if extMime := uploadExtensionMimeTypes[ext]; extMime != "" {
+		return extMime
+	}
+	if clientContentType != "" {
+		return clientContentType
+	}
+	return strings.TrimSpace(detected)
+}
+
+// FileURLName returns the public URL file name for a stored file.
+func FileURLName(fdef *types.FileDef) string {
+	if fdef == nil {
+		return ""
+	}
+	fname := fdef.Id
+	if ext := ExtensionByFilename(fdef.OriginalName); ext != "" {
+		return fname + ext
+	}
+	return fname + ExtensionByMimeType(fdef.MimeType)
+}
+
+// ExtensionByFilename returns a safe lower-case extension from a client-provided file name.
+func ExtensionByFilename(name string) string {
+	_, fname := path.Split(strings.TrimSpace(name))
+	ext := strings.ToLower(path.Ext(fname))
+	if len(ext) < 2 || len(ext) > 16 {
+		return ""
+	}
+	for _, r := range ext[1:] {
+		if (r < 'a' || r > 'z') && (r < '0' || r > '9') {
+			return ""
+		}
+	}
+	return ext
+}
+
 // ExtensionByMimeType returns a stable URL extension for uploaded media.
 func ExtensionByMimeType(mimeType string) string {
 	switch strings.ToLower(strings.TrimSpace(strings.Split(mimeType, ";")[0])) {