dlna: set more correct mime type

The code currently hardcodes `text/srt` for all subtitles.

`text/srt` is wrong, it seems `application/x-subrip` is the official
extension coming from the official mime database, at least (and still
works with the Samsung TV I tested with). Also add that one to `fs/
mimetype.go`.

Compared to previous iterations of this PR, I dropped tests ensuring
certain mime types are present - as detection still seems to be fairly
platform-specific.

cmd/serve/dlna/cds.go

@@ -101,9 +101,24 @@ func (cds *contentDirectoryService) cdsObjectToUpnpavObject(cdsObject object, fi
 			Host:   host,
 			Path:   path.Join(resPath, resource.Path()),
 		}).String()
+
+		// Read the mime type from the fs.Object if possible,
+		// otherwise fall back to working out what it is from the file path.
+		var mimeType string
+		if o, ok := resource.DirEntry().(fs.Object); ok {
+			mimeType = fs.MimeType(context.TODO(), o)
+			// If backend doesn't know what the mime type is then
+			// try getting it from the file name
+			if mimeType == "application/octet-stream" {
+				mimeType = fs.MimeTypeFromName(resource.Name())
+			}
+		} else {
+			mimeType = fs.MimeTypeFromName(resource.Name())
+		}
+
 		item.Res = append(item.Res, upnpav.Resource{
 			URL:          subtitleURL,
-			ProtocolInfo: fmt.Sprintf("http-get:*:%s:*", "text/srt"),
+			ProtocolInfo: fmt.Sprintf("http-get:*:%s:*", mimeType),
 		})
 	}
 

fs/mimetype.go

@@ -30,7 +30,7 @@ func init() {
 		{"video/webm", ".webm"},
 		{"video/x-msvideo", ".avi"},
 		{"video/x-matroska", ".mpv,.mkv"},
-		{"text/srt", ".srt"},
+		{"application/x-subrip", ".srt"},
 	} {
 		for _, ext := range strings.Split(t.extensions, ",") {
 			if mime.TypeByExtension(ext) == "" {