If you have Xcode installed there is a nifty tool called
plutil to convert plist files from binary to xml and vice versa.
Why do you care?
Well if you are doing unadvertised things to your plist files you may want to verify the contents, and if its in binary form, then it’s a little hard to figure out what is going on.
So you can convert it to xml with
$ plutil -convert xml1 path/to/plist/file
and then to convert back to binary
$ plutil -convert binary1 path/to/plist/file
In my case, I was setting Mail to use plain text by default
$ defaults write com.apple.mail PreferPlainText -bool TRUE
and wanted to verify the change.