input type file with autopostback DropDownList

I have a form to upload files with <input type=file>... This form also has a dropdownlist with autopostback set to true a few textboxes and a upload button. When the user selects a value with the dropdownlist it goes to the server and updates part of the form the problem is this also sends the file specified in the input I want to prevent this and only send the file when the user clicks on the upload button. Any ideas?
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