Also, give a "Browser not supported" error for browsers which do not support
the Blob constructor.

This script adds a single download script, 'file.php', which provides access
to files of all types to all users. It uses a HMAC signature system with an
expires time, which allows signed URLs to be sent to external users.

This also fixes an issue with the Http::cacheable() method, where the
last-modified and Etag headers were not properly compared, which resulted in
permanent cache misses by the client.

Only one file is supported at a time now; but at least the files are saved
and displayed in the event of validation errors.