IFF Anims starting with audio don't show video

[MaddTheSane]

Summary of the bug:
If an IFF file starts with an 8SVX structure, FFMpeg thinks the whole file is an audio file and decodes even the video portions as audio, resorting in static for a good portion of the audio as it is decoded as audio.
How to reproduce:

% ffmpeg -i At_the_Movies.anim -c:a pcm_s8 atm1.aiff
% ffprobe At\ the\ Movies\ 1.anim 
ffprobe version 3.2.2 Copyright (c) 2007-2016 the FFmpeg developers
  built with Apple LLVM version 8.0.0 (clang-800.0.42.1)
  configuration: --prefix=/opt/local --enable-swscale --enable-avfilter --enable-avresample --enable-libmp3lame --enable-libvorbis --enable-libopus --enable-libtheora --enable-libschroedinger --enable-libopenjpeg --enable-libmodplug --enable-libvpx --enable-libsoxr --enable-libspeex --enable-libass --enable-libbluray --enable-lzma --enable-gnutls --enable-fontconfig --enable-libfreetype --enable-libfribidi --disable-indev=jack --disable-outdev=xv --enable-audiotoolbox --enable-sdl2 --mandir=/opt/local/share/man --enable-shared --enable-pthreads --cc=/usr/bin/clang --enable-vda --enable-videotoolbox --arch=x86_64 --enable-yasm --enable-libx265 --enable-gpl --enable-postproc --enable-libx264 --enable-libxvid
  libavutil      55. 34.100 / 55. 34.100
  libavcodec     57. 64.101 / 57. 64.101
  libavformat    57. 56.100 / 57. 56.100
  libavdevice    57.  1.100 / 57.  1.100
  libavfilter     6. 65.100 /  6. 65.100
  libavresample   3.  1.  0 /  3.  1.  0
  libswscale      4.  2.100 /  4.  2.100
  libswresample   2.  3.100 /  2.  3.100
  libpostproc    54.  1.100 / 54.  1.100
[iff @ 0x7fd367005800] Estimating duration from bitrate, this may be inaccurate
Input #0, iff, from 'At the Movies 1.anim':
  Metadata:
    comment         : TOROID
  Duration: 00:01:21.64, bitrate: 72 kb/s
    Stream #0:0: Audio: pcm_s8_planar (ANIM / 0x4D494E41), 9085 Hz, mono, u8p, 72 kb/s
% file At\ the\ Movies\ 1.anim 
At the Movies 1.anim: IFF data, ANIM animation

[Carl Eugen Hoyos]

Is this issue not reproducible with current FFmpeg git head?

[MaddTheSane]

It is.

[Carl Eugen Hoyos]

Sample from ​http://www.randelshofer.ch/animations/anims/eric_w_schwartz/At_the_Movies.animVE.html

[MaddTheSane]

Originally from ​http://www.randelshofer.ch/animations/anims/eric_w_schwartz/AmyDoesSchwab.animVE.html

[MaddTheSane]

I attached a simpler, smaller animation with only one sound sample instead of 15.

[MaddTheSane]

This issue is still present.

[MaddTheSane]

Sample from ​http://www.randelshofer.ch/animations/anims/eric_w_schwartz/ATMFinalVE.html

[MaddTheSane]

The attached file "At The Movies 2.anim" contains audio, but the first substruct of the ANIM form is ILBM, not 8SVX.