attaching a patch does not enable the "is-patch" knob #241

Attaching a file named some.patch does create an attachment with the mime type text/x-patch, but apparently this does not enable the Patch property.

Is this something the Bugzilla instance should do automatically, or is this something python-bugzilla needs to do?

Command was bugzilla --bugzilla $URL --ensure-logged-in attach --description some.patch --file some.patch $Bugnumber.