Googling around for that error, apparently the bare-bones stock Android OS may refuse to download any files of a type it doesn't recognize. This may be happening because it's expecting a MIME type to be specified by the server rather than going by the filename extension; we serve all downloads with the MIME type="binary/octet-stream" (i.e., raw binary data) to help ensure that browsers will actually download the file rather than trying to display it inside the browser or launching an associated app to open the file without actually downloading a copy to keep.
The only workarounds I could find suggested installing a download manager or file manager app such as Astro, which can make the OS accept any file type, or using another browser app such as Opera rather than Android's built-in stock browser. We haven't received any reports of this error before, possibly because most cell carriers bundle their Android phones with a suite of apps including an app that can view JPGs (such as pics created by the phone's camera) and/or a nicer browser app.
One of our Developers here has an Android phone and used that to test one of your download links; upon clicking the link, her phone gave the message, "The Content is not supported on the phone. No application can be found to open this file. Do you still want to download it? [YES] [NO]" At least that still allowed downloading the file, so the exact message and behavior may vary among different Android builds, cell carriers, and models of phone.