avformat/flac_picture: print a warning when mimetype is unknown

It's not an error since bba6df9a.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/flac_picture.c

@@ -89,7 +89,7 @@ int ff_flac_parse_picture(AVFormatContext *s, uint8_t **bufp, int buf_size,
         mime++;
     }
     if (id == AV_CODEC_ID_NONE) {
-        av_log(s, AV_LOG_ERROR, "Unknown attached picture mimetype: %s.\n",
+        av_log(s, AV_LOG_WARNING, "Unknown attached picture mimetype: %s.\n",
                mimetype);
         return 0;
     }