Ask about changing filename instead of silently adding an extension.

[aculich]

For example, a file named .cvimrc automatically gets renamed as .cvimrc.txt and
there is no way to override that with gist-supported-modes-alist to have no
extension since a . always gets appended.

gist.el/gist.el

Line 210 in 314fe6a

(proposal-fname (concat (file-name-sans-extension name) "." ext))

If the file already has a name, then just use it! If it is a buffer with no file
associated then it's worthwhile guessing/asking about the filename, but don't
otherwise change it.