You have to POST the file to this URL (the name of the form field have to be “file”)
http://yourdomain.com/index.php?route=tool/upload
If everything was ok, you will get similar response:

{
"code": "841c0e0f9a0a1460d17a7368471d8121b579435b",
"success": "Your file was successfully uploaded!"
}

Then you have to POST the code as the value of your custom field when you register a new user.