[package:mime] make mp3 default for audio/mpeg

[fstotz]

#2043

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[devoncarew]

Hi @fstotz, thanks for the contribution!

Re: changing the default extension for mpeg - we're considering updating this package to use another canonical mapping (Apache httpd?), and then having few if any diffs from that mapping. If we thought a mapping needed to be updated, people could either update the new system-of-record, or have local changes for their project, but for the most part we wouldn't change this project. See also https://github.com/dart-lang/tools/issues/2028.