Giving a try to the current gaupol from git in a live session of Fedora 24 Workstation:

• Create a new document, then trigger the "save" action: the save dialog is opened.
• Write some name for the file, without to specify any extension. Note here that the "SubRip" format is selected by default in the respective drop-down menu.
• Still having the focus in the filename box, press Enter. The file is created with the given name without any extension; If I would chosen to click in the Save button to confirm the action, the created file would have the extension .srt, as it corresponds to the selected format.