.dat - Unspecified data
Files with the .DAT extension can really be anything, but it is usually designed to be read by one application only.
The most popular use of .dat files on the Mac may be FireFox which use .dat files for its cache
Another example is Nero BackItUp for Windows which package the backed up data in .dat files. They are supposed to be restored by an application called NRESTORE.EXE, but this may not always work. The backed up files are not compressed or encrypted inside the Nero Backup file, and File Juicer will find and extract images from such a file just fine.
Some backup applications make incremental backups, or use proprietary compression algorithms, but since JPEG files are already compressed they can be left inside the backup archive as they are, and therefore be recovered by File Juicer. The same applies to PNG and other popular media formats.
Winmail.dat files are sent by misconfigured Outlook servers, and File Juicer may extract some, but not all, files from those. You may redirect an email with one of these files to a google mail account, and you may have luck that gmail can decipher it.