When uploading a file using fileUpload (button upload, etc.), the progress meter position is calculated based on a status ping happening every seconds, in which the server report the current progress (current size uploaded / total file size).

Some network configurations prevent the server from replying properly, and this causes the progress bar to be non-indicative / confusing for the end user.

• If a firewall with file analysis (virus, integrity) caches the upload until the whole file is available for analysis, the web server will receive the whole file in one transaction. (This will cause the progress bar to show results such as: [0,0,0...,0,0,100].
  -If a user upload a file from a network location in Firefox, the browser cannot send / receive other network requests during the file transfer. This causes the progress bar not to open, and the client side feel unresponsive. (upload still happen, but the UI "freezes" until it is completed).

In this case, an option for fileUpload to use the client-side progress information instead could be useful.
onUpload="[other options],clientProgress=true"