On Loki I have the same problem with several applications which involve a file dialogue. When I select a file there is no way to 'Save', as there is no button to click, [Enter] doesn't work and (double-)left-click doesn't work. Could you look at the picture below and tell me whether there is a button missing from this standard file dialogue? Perhaps a header bar is missing that should have Save, Cancel and Search buttons.
'Pantheon-files' had no header bar so no 'Cancel', 'Save' or 'Search' buttons! I removed 'pantheon-files' and installed 'Nautilus' which fixed it. I have since installed Juno - which had the same problem and the same solution.