Serve correctly ebook reference files

Summary:
On Android systems, .epub ebooks are donwloaded as .txt files,
following the default text/plain type currently configured for nginx.

As the default nginx configuration for MIME types has been imported in D3683,
it's trivial now to add sections for other known MIME types to address this.

Test Plan: Download on Android, saved as .epub, not .txt

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3684