Use MIME::Types to infer the correct content_type

These case statements were duplicated and didn't provide anything
useful over the provided MIME::Types module.
Add @KushalP to the list of CONTRIBUTORS

I should've added myself after #443, so here I am.
Add a runtime dependency on mime-types gem
Merge pull request #492 from KushalP/simplify-mime-type-inference

Simplify mime type inference