convert mimetype_fu plugin to proper gem

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Change-Id: I633d3e9849cb750010591bc6a46897b3f148bb3e
Reviewed-on: https://gerrit.instructure.com/29467
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Stephan Hagemann <stephan@pivotallabs.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>