How to change/customize mail signature in Mountain Lion? To add an custom mail-signature in Lion you had to add an signature via preferences, and then replace the contents of the webarchive with your own.

In Mountain Lion, Apple replaced the webarchive format for signatures with an new format: .mailsignature. When I open such an file, I just see HTML (!). But when I change it, it doesn't take effect.. 

Does someone have an solution to perform this?
Quit Mail, edit the .mailsignature file with a text editor changing the HTML to whatever you like then right click the file in Finder and select "Get Info" then tick the "Locked" checkbox.  This prevents Mail from resetting the file to the original version and it uses your version.