Add support for MPEG-4 Audiobook (.m4b)

[Borewit]

Fixes #165
Related: #99, #60, #38

Note that I removed check([0x4D, 0x34, 0x41, 0x20]), at offset 0. I don't think this picking out any MPEG-4 audio file.

[sindresorhus]

Tests are failing as the audiobook fixture is being detected as .mp4.

[Borewit]

Not sure if it is actually that simple to pick out the .m4b extension.

[sindresorhus]

@Borewit Still interested in finishing this? :)

[Borewit]

You may close the issue.
I don't think there is a feasible way to extract the audio-book type.
Although this can be found in the metadata, the complexity to get that out with the risk it might not be within the first 4k, I don't think it's worth to implement it.

I started this PR based on the assumption it could be extracted with the first bytes, which appears not to be the case.

I went through the changes of this PR, and kept a minor code correction, which I moved to PR.

I use this module for music-metadata to determine the audio file type, based on the content, as a fallback in case the audio type wasn't provided by the user. As long as I get one of the MPEG-4 MIME-types it works for me.

Feel free to ping me on any discussion regarding audio file type, or on requests to cover new audio file types.

[sindresorhus]

I understand. No worries at all :)

Feel free to ping me on any discussion regarding audio file type, or on requests to cover new audio file types.

Will do.