Allow binary files to be uploaded

This fixes https://github.com/gollum/gollum/issues/737

It's not a particularly smart implementation, but it should be fairly future-proof since it doesn't require any enumeration of text or binary formats.

There are no tests for adding binary files, and I don't understand Gollum's testing strategy well enough to add one, but I'd be willing to pair with someone knowledgeable on it.