As far as I can tell, this is a protocol/server issue.
http://en.wikipedia.org/wiki/AppleSingle_and_AppleDouble_formatsMac OS X revived the use of AppleDouble; on file systems such as NFS and WebDAV that don't natively support resource forks, Finder information, or extended attributes, that information is stored in AppleDouble format, with the second file having a name generated by prepending "._" to the name of the first file (thus, this information acts as a hidden filewhen viewed from a non-Apple Unix-based operating system).
Were you expecting to pass a url to a server, such as http://host/application/Mail.app and have it download the whole AppleDouble package? It would depend on the server to somehow convert that into a single file and then the client knowing how to re-assemble it when it received it. At this point, Sardine doesn't handle the application/applefile MIME type (or any other MIME type) specially. It simply gives you an InputStream of data. Thus, if you can find a WebDAV server to send you the data properly, you should be able to write something to decode it. =)
jon