avformat/id3v2: detect PNG by header instead of mime

the mimetype for PNG can be set to jpeg

Fixes 01\ -\ Cider\ Time.mp3
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/id3v2.c

@@ -1083,6 +1083,9 @@ int ff_id3v2_parse_apic(AVFormatContext *s, ID3v2ExtraMeta **extra_meta)
         st->codec->codec_type = AVMEDIA_TYPE_VIDEO;
         st->codec->codec_id   = apic->id;
 
+        if (AV_RB64(apic->buf->data) == 0x89504e470d0a1a0a)
+            st->codec->codec_id = AV_CODEC_ID_PNG;
+
         if (apic->description[0])
             av_dict_set(&st->metadata, "title", apic->description, 0);