Conform to Refile Ruby gem

[sineed]

These changes make possible to use exfile alongside refile (tested with version 0.6.2). I don't think it is a good idea to merge this to master (refile use sha1 encryption instead of sha256). Maybe it can be useful for someone else. In my case I already have an existing Rails app with Refile gem used and want to read/write files in new Phoenix app from the Refile's backend.