Is there a way to prevent or cancel an attachment?

When an Outlook user attempts to attach a file to a mail item, I would like
to be able to programmatically decide whether the attachment meets certain
criteria and either allow or disallow the attachment process to continue.

I've seen code which puts up a warning message box when the AttachmentAdd
event fires, but is there a way to prevent the attachment entirely?  Perhaps
an event that fires *before* the attachment is added?

If all else fails, is there a way to hook into the attachment selection
dialog box?  I know that Outlook puts up its own dialog, as opposed to
calling the Win32 GetOpenFileName API function.  Is this dialog something
that I can detect and intercept using an addin?

Any help or suggestions would be greatly appreciated.

Thanks

---jkb