Triggered on upload progress. Not available for legacy IFRAME upload operations.
The event object receives the following additional properties:
files- a list of File objects in the current selection operation. Each file contains:
name- the name of the file
size- the size of the file in bytes
type- the file's MIME type
percentage- the percentage of the upload progress - between 0 and 100
listItem- the <LI> element displaying the item