Alter form input string

I have a form that posts into my blog. It has a file upload field that users upload their movies with. My webserver formats the video file to HTML5 compatible format and to display that video to visitors it needs to be in the format [video src="http://www.mysite.com/video"]. But the gravity form file uploader posts the complete URL like http://www.mysite.com/video.mov.

how can I alter the URL that the file uploader creates to the shortcode I need it to be? Is gform_field_input the right way? An example would ofcourse be very much appreciated. :)

Thanks

Thanks for the reply,
But will this mess up the file uploading process?

Either way I could not get it to work. I did the following on form_display.php
...
else{
//pre submission action
do_action("gform_pre_submission", $form);
do_action("gform_pre_submission_{$form["id"]}", $form);
do_action("gform_pre_submission", "pre_submission_handler");

function pre_submission_handler($form){
$_POST["input_6"] = "new value for field 6";
}

Shouldnt I see "new value for field 6" in my post?