An important lesson about PHP APC RFC 1867
If you don't know -- although I don't know how you found this if you do -- APC stands for Alternate PHP Cache, and RFC 1867 is a sort of stopgap way of fetching the progress of a file upload.
Here's something I learned after a few hours of frustration, trying to put an upload progress bar into the Contact Enhanced module for Joomla for a client:
The APC_FILE_UPLOAD field MUST come sooner in your form than the file field.
Try it yourself!