AIFF mimetype: fix sniffing rule.

AIFF files follow EA IFF spec and start with a FORM chunk. They never
start with AIFF or AIFC directly.

See for example:
http://www.onicos.com/staff/iz/formats/aiff.html
http://paulbourke.net/dataformats/audio/
Fix AIFF decoding with ffmpeg.

AIFF files are little endian. ffmpeg detects this just fine, but we
overwrote the endianness in the decoded format info with
B_MEDIA_HOST_ENDIAN right after setting it.

Now we can decode both AIFF and WAV files just fine.

Fixes #7047.