XML
XML Files are mostly readable with a text editor like TextEdit or TextMate, but they can contain data in any file format in CDATA sections.
If this data is a JPEG file (or any of the other formats from File Juicers preferences), File Juicer can extract it.
Some XML files does not use the extension XML - but something uniquie for the application which use it. Examples are KML used by Google Earth or XMP used by Adobes software.
iTunes and iPhoto are examples of applications which save some of their album information in XML to make it more available for other applications. Apple's iWork applications use compresed XML internally.