Steven Shockley points out that the .PPK extension is far from obvious to

Steven Shockley points out that the .PPK extension is far from obvious to
users. Update the file selection dialogs to mention it per the usual Windows
convention, and also sprinkle references to it throughout the docs. I've
also scattered hints that most tools need PuTTY's native format; perhaps this
will reduce the frequency with which FAQ A.1.2 trips people up.

[originally from svn r2625]