Do this by setting disabled="true" on the file input element after
selecting the file, and re-enabling it if the file is removed. This
prevents the file from actually being sent to the server, since we're
not using it there anyway.
- Populate the POSTed form with additional data from the existing TO
model. This way the pdf attachment has to be explicitly overwritten.
- Adjust the frontend so that if there is an existing PDF, it only sends
a file input back if the user removes the existing PDF.
