append_form_string
Description:
Adds a new MIME part containing data to this, using "Content-Disposition: form-data", as per the
      HTML forms specification.
See request_new_from_multipart for more details.
Parameters:
| this | a multipart (presumably of type "multipart/form-data") | 
| control_name | the name of the control associated with  | 
| data | the body data |