avformat/matroskaenc: Improve mimetype search

Use the mime_types of the corresponding AVCodecDescriptor instead of
tables specific to Matroska. The former are generally more encompassing:
They contain every item of the current lists except "text/plain" for
AV_CODEC_ID_TEXT and "binary" for AV_CODEC_ID_BIN_DATA.

The former has been preserved by special-casing it while the latter is
a hack added in c9212abf so that the demuxer (which uses the same tables)
sets the appropriate CodecID for broken files ("binary" is not a correct
mime type at all); using it for the muxer was a mistake. The correct
mime type for AV_CODEC_ID_BIN_DATA is "application/octet-stream" and
this is what one gets from the AVCodecDescriptor.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -1667,17 +1667,11 @@ static int mkv_write_attachments(AVFormatContext *s)
         if (t = av_dict_get(st->metadata, "mimetype", NULL, 0))
             mimetype = t->value;
         else if (st->codecpar->codec_id != AV_CODEC_ID_NONE ) {
-            int i;
-            for (i = 0; ff_mkv_mime_tags[i].id != AV_CODEC_ID_NONE; i++)
-                if (ff_mkv_mime_tags[i].id == st->codecpar->codec_id) {
-                    mimetype = ff_mkv_mime_tags[i].str;
-                    break;
-                }
-            for (i = 0; ff_mkv_image_mime_tags[i].id != AV_CODEC_ID_NONE; i++)
-                if (ff_mkv_image_mime_tags[i].id == st->codecpar->codec_id) {
-                    mimetype = ff_mkv_image_mime_tags[i].str;
-                    break;
-                }
+            const AVCodecDescriptor *desc = avcodec_descriptor_get(st->codecpar->codec_id);
+            if (desc && desc->mime_types) {
+                mimetype = desc->mime_types[0];
+            } else if (st->codecpar->codec_id = AV_CODEC_ID_TEXT)
+                mimetype = "text/plain";
         }
         if (!mimetype) {
             av_log(s, AV_LOG_ERROR, "Attachment stream %d has no mimetype tag and "