DFP adds several data bindings to the page to assist in debugging file upload issues. If you open the Bindings panel on your upload page, you should see two entries in the Bindings panel:
"WA Upload Files Global Status" and when expanded, a "Debug Info" item within it.
"WA Upload Files (fileFieldName)" and when expanded, a number of entries, including a "Debug Info" entry.
If you drag the Debug Info item onto your page, and then attempt the upload, you may see more info about what info PHP has on the file that was uploaded (or rather attempted to be uploaded).
You could also post a link to a page that exhibits this issue. Perhaps there is a problem with the coding of the form itself that is causing this issue.
Beyond that, we do have free support, so please do submit a support incident if you need to.