gh-120258: Correct MIME type for .ass files

[rcombs]

Advanced SubStation is a common subtitle format, and the ".ass" extension is not commonly used for AAC audio files.

• Issue: .ass files give incorrect MIME types #120258

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[aisk]

Hi, according to the devguide, only trivial changes like fixing a typo don't require a GitHub issue first, so I think this change requires a related issue.

[rcombs]

I'd thought of this as sufficiently trivial, but sure, done.

[aisk]

I did some research and found that although .ass subtitle files are more common these days, .ass is also an IANA registered file type for the audio type: https://www.iana.org/assignments/media-types/audio/aac. So, I think we can't change it directly.

Moreover, the mimetypes package not only uses this predefined map in the source code, it also uses the system's mimetype database to get the associated mimetypes from the file extension, and Debian uses data from IANA: https://salsa.debian.org/debian/media-types/-/blob/master/mime.types#L1681