The allowable file type "exx" is actually a naming convention placeholder for the disk image file type that the Encase Forensics software outputs. The actual extension is ".e01", if the image is less than 640 MB. If it is over 640 MB, the first 640 will be stored in a file with the extension ".e01", and the next 640 will be ".e02" and the next 640 ".e03" and on and on (see http://www.forensicsware.com/blog/e01-file-format.html). At the very least we can change "exx" to "e01" for the time being-- and perhaps think about providing support for uploading serialized .e01, .e02, .e03, etc. files into one Fedora object (maybe as a .tar or .zip file?) at a later date?