This control allows to watch status of files uploading to server. You can see what part of uploading content is currently received by server, what file is currently uploading, what files have been uploaded already, current speed of data transmitting, elapsed and estimated time.

You can control uploading process after submitting.
Allowed file extensions and maximum sizes can be set for uploading files. On submit file inputs that contain files with not allowed extensions are cleared and no submit occurs. Files with size exceeding maximum are rejected from HTTP request and server-side does not receive them.
See also working example Allowed file extensions and sizes.

More than one worker process can be set in IIS for File Upload Progress version 1.8.1219.1 and up.