Added additional mapping

[ajstanley]

Link to Github issue or other discussion

Issue 393

What does this PR do?

Allows mp3s with audio/mp3 to be ingested

What changes were made?

Adds additional mimetype mapping

How to test / verify this PR?

Ingest mp3 from S3

Interested Parties

@mjordan

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Checklist

• Have you run pycodestyle --show-source --show-pep8 --ignore=E402,W504 --max-line-length=200 yourfile.py?
• Have you included same configuration and/or CSV files useful for testing this PR?
• Have you written unit or integration tests if applicable?
• If the changes in this PR require an addiional Python library, have you included it in setup.py?
• If the changes in this PR add a new configuration option, have you provided a default for when the option is not present in the .yml file?

[ajstanley]

I'll make this configurable when I get a chance, so anyone encountering a weird mimetype can add their own extension, but this fixes the problem with the s3 headers.