In Linux Mint (and Ubuntu, other Linux distributions and various competing operating systems), it is frequently possible to open a data file in an appropriate program by double-clicking the icon for the file, either on your desktop or in a file browser. This is handled by associating the file's extension (which Linux treats as a MIME type) with the corresponding program. In Linux, the association is done by the file manager. The distributions I've used in recent years (Ubuntu and Mint) have been based on the GNOME desktop, which in turn uses the Nautilus file manager.
Many programs set up file associations at the time you install them. For example, the LibreOffice productivity suite associates itself with OpenDocument file extensions (.odt etc.). Many programs, however, do not do that automatically; in some cases, it is because they claim no specific file extensions. In those cases, Nautilus offers the option to right-click a file of the desired type (extension), click Properties > Open With, select the application (clicking Show other applications if necessary to find it), and Add the application to the list of default choices for that file type.
This works fine as long as Nautilus lists your application of choice. I ran into a problem with that today. I was trying to make SQLite Studio the default application to open files with the extension .sqlite. (Incidentally, if you are looking for a program that will let you create, browse and modify SQLite databases, I definitely recommend SQLite Studio. It's available on Linux, Mac OSX, Windows and a few other operating systems.) Unfortunately, it did not appear on the application list when I clicked Show other applications. This appears to be the result of a bug in current versions of Nautilus.
Two things are necessary to have your application display in the list. First, there needs to be a .desktop file for it, typically residing either in /usr/share/applications or in ~/.local/share/applications. In my case, the .desktop file already existed: I had added SQLite Studio to the start menu, which automatically creates a .desktop file in the latter directory. So why was it not appearing on the list??
After considerable searching, I found the answer here: the command line must end with "%u" (which is the placeholder into which Nautilus will insert the name and path of the file to be opened). I suppose this makes sense in hindsight; without that placeholder, the user might be selecting an application that does not accept a file to open as a command line argument. Still, I would have thought Nautilus would just append the file name to the command line, with or without the placeholder.
You can add the placeholder either by editing the .desktop file directly, in a text editor (append it to the "Exec=" line) or by right-clicking the start menu, selecting Edit Menu, finding the application, clicking Properties and editing the Command field. Once you have a .desktop file containing the "%u" placeholder, the application should appear on the Nautilus list of available applications.