clicking cancel on a file already uploaded using the stock upload widget makes it appear the file is cleared when it is not and it gets uploaded to the task on completion.
either really clear out the file or if it's not cleared out show that it's not cleared out. But you would need to provide some way to get rid of the file.
Steps to reproduce the issue:
Put an upload widget on a form
upload a file on to the form
click on the upload widget again
it appears the file has be cleared out
and there is a message saying no file
complete the form and the file is uploaded to the task