In issue 655799, for cases where user clicks KEEP on downloads that are eligible for download feedback reporting, we create a temp copy of the download file and upload this temp copy to download feedback server instead of the actual download file. In this way, download files will be (almost) immediately available to user once they clicks KEEP.

As a follow-up of issue 655799, if this mechanism works out fine, we are going to refactor download feedback service code to use the similar method for discarded download too. 
Meanwhile, add more tests to verify the reliability of this upload mechanism. For example, we want to test that download file (at GetFullPath()) can be safely deleted without the upload process getting interrupted.