Step 2. Move validations to custom validator
Improve error messaging for attachments

Prior to this the user got a couple of cryptic error messages:

"file is not an allowed file type" and "file can't be blank"

This improves things somewhat, giving an error message more relavant
to the actual problem encountered.
Add flag for permissive zip file contents

So that certain users can upload zipped executables, we add a new
flag on the AttachmentData model. If set to true, the contents of the
zip file will not be examined for illigal file types.

Note that virus checking will still be performed as expected.
Wire up the controller to set the override.

We set the flag for skipping file type verification based on the
user's privileges.
Add support to conflicting controllers

We have to set the filter manually for the corporate information pages
and supporting pages controllers because they do not include the
Attachments module directly.
Simplify filter logic.

I did consider extracting this logic out to
Admin::AttachmentActionParamHandler, but it's probably concise enough
as it is.