Binary package hint: nautilus
When accessing a FAT volume, all the files have the execute bit set. When you double click (or right click and choose "Open") a file that has a text MIME type, nautilus assumes it is a script and offers to run it (or run in a terminal or display it), regardless of what application is assigned to actually open the file.
Use case: an SRT file is stored on a USB memory stick that comes formatted as FAT out of the factory. The user has Gnome Subtitles set as the default application for SRT files (which are a type of text file). The user plugs the USB memory stick into the computer and double clicks the SRT file and expect Gnome Subtitles to open the file. Instead they get a "Run or Display" dialog.
In such a case, even the "Open With" list does not include the needed application - if the user wants to open their SRT file they must right click the file, choose Open With->Other Application, search for Gnome Subtitles and select it. Of course Nautilus does not actually "remember" this setting and the user has to do this again for the next file.
Thank you for your bug report. The issue is an upstream one and it would be nice if somebody having it could send the bug the to the people writting the software (https:/