Chain %d->%F->%f in the attachment menu.

Chain %d->%F->%f in the attachment menu.

Previously, %d would use %f if there was no description set.

Place the new %F option in between %d and %f.  This way, %d will fall
back on %F, which will fall back on %f.  This allows the standard
attachment menu to show d_filename.

This is useful for forwarding attachments or editing draft files with
attachments.  In these cases the actual filename is sanitized but the
attachment name is preserved in d_filename.