Adds a new MIME part containing body to this
Uses "Content-Disposition: form-data", as per the HTML forms specification.
| this |
a multipart (presumably of type "multipart/form-data") |
| control_name |
the name of the control associated with this file |
| filename |
the name of the file, or null if not known |
| content_type |
the MIME type of the file, or null if not known |
| body |
the file data |