Can't paste from files with .arc extension

[Brian Adkins]

This seems like a strange problem to me. For some reason, I can't
select portions of a file with the mouse and paste it in another
window, if the file in question has a .arc extension.

I've been experimenting with Paul Graham's Arc language which as .arc
extensions. I originally thought it might have something to do with
the arc-mode.el major mode file I've been using for Arc code, but I've
turned that off, so the files are loaded in Fundamental mode.

I also thought it could be a file encoding issue, but if I copy the
file to a file with a different extension (.scm for example), I can
open the file and copy/paste with the mouse just fine. If I then copy
the file to a different file with a .arc extension, copy/paste is
broken again.

I can copy/paste within emacs fine, I just can't paste into another
application.

Can anyone think of why emacs might disable pasting to another window
for files with .arc extensions?