param calls won't return return anything if POST_MAX is reached. I don't know if that's what's causing the problem, but it'll surely hamper your efforts to repopulate the form.

The file upload is not sent to the web server any differently than other inputs (text fields, radio buttons, etc). If the POST body is bigger than POST_MAX, then it is not processed, discarding all inputs indiscriminately.

Update: A workaround would be to input the generic data on one page, the upload the files from another page.