This fixes files disappearing when uploading multiple files. Only one file
at a time is uploaded to the server.

This patch (which needs further development), introduces an extra field to
the %attachment table to store a file's name, in the event that two files
with the same content are stored in the database with differing names. In
such a case, the name is stored in the %attachment table. This allows the
attachment to specify a different name each time the same file is attached
to something in the system.

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.