as requested by the arduino folks:
http://processing.org/discourse/yabb_beta/YaBB.cgi?board=os_core_pde;action=display;num=1184589983

dave, did you have ideas on how this should be implemented? i'm doing major
work on the editor/environment so i can get a fix in there if you'd like.

(also not sure if this is the right email for you, it's what you had used
on two very old bug reports)

I don't have any fleshed out ideas, but I imagine something like:

String defaultExtension = ".pde";
String[] codeExtensions = { ".pde", ".java" };

Or, in the case of Arduino:

String defaultExtension = ".pde";
String[] codeExtensions = { ".pde", ".h" ".cpp", ".c" };

and then simply iterate through the arrays any time you'd otherwise use the literal extension.
Would you like me to take a shot at this?

if you have a moment, let me know--that'd be great. if not, i'll try to get
to it sometime soon.

This was way more work than i had expected, but i've implemented it for
0147. I was in the midst of cleaning up the API for Editor, Sketch, and
SketchCode to make way for the user-contributed Tools menu. Man, that code
is ugly in places.