Create a fake mimemagic gem in the vendors folder [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Related to the incident: #325851 (closed)

We can not use the mimemagic gem, as it is now GPL, so we create a fake gem and use that instead.

Dependent on removing our internal mimemagic usage: !57387 (merged)

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