Hi there!
The “save as” dialogue is missing the file type selection option. I writer there is no way to save a document as a .doc or in any format other than .odt. Similarly in calc etc.
My OS is Ubuntu 16.04. The problem occurs both with the default version (5.1.6) and with the “fresh” version (5.3.3, installed using PPA).
I remember dealing with the same problem years ago, also on Linux. Probably it was still named OpenOffice then. Somehow it got solved but I don’t remember how. Please help.
In the save as dialog, in the lower right you should see a drop down box All Formats. Click on the down arrow & select the format you want.
Ctrl+Shift+s (keyboard shortcut for “Save As” dialog). Then do what Ratslinger said 
Thank you guys! This works.
How silly of me. I was used to this button controlling only which files appear in the frame above it, while there was a separate button for controlling the file format. This way of organizing the dialogue box actually makes a lot of sense.
Cheers!