Fixes #1822

Findings

getAudioTypesOnly was missing from MimeTypes. This functionality is now required by the Audio Block in Mobile Gutenberg because the Gutenberg Bridge's integration with the native media pickers need to filter media content by the audio MIME types only.

Solution

To add this behavior to MimeTypes

Testing

A unit test is included in this solution that verifies that the getter works appropriately. For manual tests, you can create an instance of MimeTypes and notice getAudioTypesOnly

Reviewing

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Submitter Checklist

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.
• I have considered adding accessibility improvements for my changes.
• If it's feasible, I have added unit tests.