Added extra table for mimtype if os level check finds no match

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What does this MR do?

This MR adds an extra table of mime-types to search, if the golang table (used in the function mime.TypeByExtension) fails to find a match. If this table also fails to find a match, the code falls through to the existing os level tables.

This table can be used to insert mime-types that are not in the golang table while providing consistent os independent behavior. In particular it is being used to address an inconsistency found in #1103

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines

As an easy to understand trivial (~5 lines) of code change, I did not create unit tests, acceptance tests or documentation updates. These can be added if the reviewers wish it.