We've created a new request type that requires the Summary and Attachment fields and then links to a custom form. When the Attachment field is set to Required, the form throws this error:
'Required' field is not linked
Link a field in this form with the Attachment Jira filed in the Request new purchase order portal form.

I can't figure out how to link the Attachment field in the form, it is not an available field in the LINKED JIRA FIELD drop down.

In order to work around the problem, I've had to disable the Attachment field as a required field in the request type:
