Store additional metadata, filename, size, etc...

See also #32 and #8

This is a work-in-progress PR that I'm sharing early for feedback. I'm trying to get Refile to store file metadata, namely filename, size and content type in the DB alongside the id. This is particularly tricky because it **must** work with all of Refile's existing features:

- [x] Direct and presigned uploads
- [x] Uploads from URLs
- [x] Form redisplays
- [ ] Extension/content_type validations
- [x] ActiveRecord integration
- [x] Removing attached files