Action Provider get-files: check file existence before returning attachment when using a *.<extension> pattern

When using the get-files action with a 'with': {'pattern': '*.txt'}, (.txt extension given as an example), if there are files with .txt extension in the directory, attachment string will be returned correctly. However, if no file is found, a unique ::attach{type_}::/$(pwd)/*.txt is returned.

We need to check that the file actually exists before returning it.