fix(HLS): Correctly identify webvtt subtitles with cmft extension (#7791)

Fixes #7766

Author: avelad

lib/hls/hls_parser.js

@@ -2935,6 +2935,16 @@ shaka.hls.HlsParser = class {
 
     if (!mimeType) {
       mimeType = await this.guessMimeType_(type, codecs, segments);
+
+      // Some manifests don't say what text codec they use, this is a problem
+      // if the cmft extension is used because we identify the mimeType as
+      // application/mp4. In this case if we don't detect initialization
+      // segments, we assume that the mimeType is text/vtt.
+      if (type == shaka.util.ManifestParserUtils.ContentType.TEXT &&
+          !codecs && mimeType == 'application/mp4' &&
+          segments[0] && !segments[0].initSegmentReference) {
+        mimeType = 'text/vtt';
+      }
     }
 
     const {drmInfos, keyIds, encrypted, aesEncrypted} =

test/hls/hls_parser_unit.js

@@ -1730,7 +1730,7 @@ describe('HlsParser', () => {
       manifest.addPartialTextStream((stream) => {
         stream.language = 'en';
         stream.kind = TextStreamKind.SUBTITLE;
-        stream.mime('application/mp4', '');
+        stream.mime('text/vtt', '');
       });
       manifest.sequenceMode = sequenceMode;
       manifest.type = shaka.media.ManifestParser.HLS;