You don't have to rename it afterwards. RTF files are just like regular text files, all that's different is that certain sequences of characters have special meaning (just as mIRC has its own set of control codes). All you have to do is convert mIRC's control codes to the RTF escape sequences, and then you can /write them to a file with any extension you want (.rtf, .txt, etc.). The extension doesn't affect the content of the file or what you can write to it with mIRC's commands, it only changes how Windows decides what program should run a file when it's double-clicked.

Spelling mistakes, grammatical errors, and stupid comments are intentional.