I downloaded a cross-assembler that as far I can tell, i am supposed to open a .txt file with it and the assembler will create an a.out file somewhere. But Fedora only lets me open .txt files with programs in its apps list. How do I put a program in the apps list or how do I simply open a file with an application outside of Fedora's app list? Thanks so much.
You need to create a desktop-file in
~/.local/share/applications of that application
For example for a *.desktop file:
#!/usr/bin/env xdg-open [Desktop Entry] Encoding=UTF-8 Version=1.0 Type=Application Exec=/usr/bin/txtEditor %u Name=MyPrivateTXTEditor