Upload problems are typically due to firewalls/proxies/antivirus sitting between the browser and DeskNow. Some of them (especially proxies) have issues when uploading large files.

Try uploading the same file from a browser running on the same computer as DeskNow, connecting to DeskNow using http://localhost/desknow/index.html (and ensuring no proxies are used).

If this works fine, check any proxy/firewall that is used when connecting via the internet (ISPs also have 'transparent' proxies that you do not need to setup in your browser. However they generally work fine).

Also antivirus products (especially with 'Internet scanning' options or similar) can have problems handling large uploads.