(Bug1262928) Add support for filtering upload file types.

Add support for having a CSV (or other file type) filter
in the File control generated by Pieforms, and use it for
the forms for uploading users, groups and group memberships.

Note that support for the accept attribute in browsers is
patchy. This will have no effect in the Chromium and Firefox
versions current at the time of preparation, but has been
successfully tested in Chrome.

Change-Id: Ib5cf42be4e054f31086e9db9cda15473e86df7a3
Signed-off-by: Nigel Cunningham <nigelc@catalyst-au.net>