It is impossible to clear upload file input box. The only standard thing u can use is a .reset() method of the form but it is not good. It will reset all other fields. File input box is READ ONLY, however there is a trick you can do.
The trick is basically to re-create the field from scratch.
Using a jQuery:
document.getElementById("filefield_id").innerHTML = document.getElementById("filefield_id").innerHTML;
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