Prompt for files with given extensions

[Robin Neatherway <robin.neatherway@gmail.com>]

I would like my function to prompt only for files with a given
extension. I have a source file, say foo.c, and would like to load the
corresponding test results file, say foo.results. However, if there
are multiple tests for foo.c, then they may have different filenames.

At the moment I use `read-file-name` with `foo.results` as a default,
but I would like to filter the possible completions by file extension.
Is this possible?