I think it to do with some of the builtin stuff in Mac OS X. I think it is a plist file, which contains XML.

It looks like of like this:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>$archiver</key>
<string>NSKeyedArchiver</string>
<key>$objects</key>
<array>
<string>$null</string>
<dict>
<key>$class</key>
<dict>
<key>CF$UID</key>
<integer>2</integer>
</dict>
<key>DfTestModelCheckboxValue</key>
<false />
</dict>
<dict>
<key>$classes</key>
<array>
<string>DfTestModel</string>
<string>NSObject</string>
</array>
<key>$classname</key>
<string>DfTestModel</string>
</dict>
<dict>
<key>$class</key>
<dict>
<key>CF$UID</key>
<integer>5</integer>
</dict>
<key>DfLogicModelLogicSong</key>
<dict>
<key>CF$UID</key>
<integer>4</integer>
</dict>
</dict>
<data>q8BHEwUXABUABAAkCRXwAAAAAAQAAAkV+fwJFfykCReYOglpsZAAlgAAAJYA AAABAQAAAAAAAAAAAAAAAAAAAAAIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAwAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAJFoc0AAAAAAAAAAAA AAAAAAknwAAST4AAAAAAAAAAAAAAAAAAAAAAAAAAAP////8AAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAwMEAAQoAAAAAAAAAABk

etc, etc...