Hi Michael,
I believe it's possible but a bit tricky. The key is to be able to trigger upload through JavaScript. You will need to do:
1. Use LayoutTemplate to customize the upload button and give it "display:none" style attribute. So that the button exists but not visible. Run the page and find out the ID of this button by viewing HTML source. For the purpose of this post, let's say its ID is "original_upload_button_id";
2. Place another button in your form. Handle this form's onclick on client side through JavaScript;
3. Inside your button's onclick handler, perform whatever operations you would like to perform;
4. If everything looks good and you want to continue the upload, call the following code:
Code: JavaScript
var originalUploadButton =
document.getElementById("original_upload_button_id");
originalUploadButton.onclick();
That should get you going.
Thanks