iFile is an app designed to allow you to move files from computers (even between different operating systems) and your iPhone or iPad. The iFile app is available from iTunes for $1.99.
To use iFile you need to connect your iPhone or iPad to a Wi-Fi network, then open the iFile app and record the IP address it is using. Then, on your Mac, Windows, or Linux computer, open a browser window and enter the IP address as the URL. If the two can connect cleanly, an upload window is displayed. From there, select the file you want to upload to the iPhone or iPad and it will be sent. The iFile interface is very basic, relying on icons for many actions. We tested iFile between iPhone and iPad, and uploaded files from both a MacBook and a Windows desktop system. The iFile interface is not customized for the iPad, relying on the iPhone interface. We could transfer music, video, and document files from either computer to the mobile device, although iFile sometimes had trouble establishing the connection reliably. We could not transfer from the mobile device back to the computer, which is unfortunate.
There are a few similar apps available, so we recommend looking at what's out there. iFile worked, but only intermittently for us, and while it's relatively easy to use, we suspect there are better alternatives.