Linux Fedora: How to open a file with a random application

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.