This isn't the fix that was actually requested.  The fix that was actually requested was to access the uploaded files in code behind, BEFORE the PostbackTrigger control is fired.  This would allow us to execute logic before actually saving anything UNTIL the PostbackTrigger is hit.  This is a client side only feature.

totally agree with Massimiliano Mattei. My user control is responsible to validate file submission logic. I don't see in a good way to validate that logic outside that user control.

Needed for example when you have a reset button and the user never submits.  I only want to maintain the files they will ultimately get submitted.  When the user resets the form those files should and need to be deleted.  No since in waiting 20 minutes, or 4 hours.

Was this request ever implemented?  I have a similar user scenario.  I have a file import feature that allows the user to upload a tab-delimited, comma-separated, or Excel file.  The user is allowed to map their file's columns to our standard database fields.  The user is also allowed to skip 0 or more "header" lines in the file.  Therefore, once the file is uploaded, we preview the first several lines and show these lines to the user.  If the user changes the number of header lines, then I would like to preview the file again with the new setting, to show what effect their choice had.  Once they have everything the way they want, then they click the final submit button, which triggers the actual import process into our database.  I need to be able to access the UploadedFiles collection across several postbacks, until they click the final submit button.  I am currently using the 2014.1.403.40 version, thank you.

I agree, we shouldn't have to use complex workarounds for every pages with file upload...

Good idea. No reason to prevent accessing the UploadedFiles collection at any time (on a button click, dropdowlist change, file upload, whatever...).
Firing the server side event OnFileUploaded and deleting the UploadedFiles colletion ONLY if the postback was triggered from controls in the PostbackTriggers is ok and it's a "genial" implementation, but this shouldn't prevent us from accessing UploadedFiles collection at any time.