Have you considered using Appcelerator for your project? File downloads are pretty straightforward plus there are plenty more advantages to running a NATIVE app versus running your app in a web client. Contact us for more information on Appcelerator and why it is the platform we use for all solutions.

of a Plugin for downloading images or videos to local iOS device, this is based on the PhoneGap Plugin API.

Basically provide a URL and a file name and the contents will be saved to the local documents folder of the application. If you notice, the download is happening in a seperate thread to that the UI will continue to respond

The javascript code