A while back we added an escape hatch for mapping certain file extensions to mimetypes for when mimeparse messes up (specfically with `text/x-rst` and text/x-markdown`). While I was looking at this code, I realized that we go through all the trouble of using mimeparse only to throw away the result. The mimeparse code isn't doing a whole lot, but it's always better to avoid doing work if we don't need to. This change swaps those so if we're going to match based on the file extension, we do that first, and only use mimeparse otherwise. Testing Done: Ran unit tests.