If there is any hack, it will only work in a single browser, and it will
stop working as soon as the author of the browser is able to fix it.
Allowing anybody else than the user to select the file for upload would
be a serious security hole.

There is no legitimate way of using an HtmlInputFile to get a file from
the visitors computer without the visitor explicitly selecting the file
for upload.

The file will be sent to the server when the form is posted. You have to
take care of the file data at that moment, otherwise the user has to
upload the file again.