I am somewhat aware of how the default grammar is picked for a filetype (correct me if I am wrong, but I believe it is the grammar chosen when the last file of that type was saved).  So I recently edited a Latex file with an extension of .txt, and now .txt files open with the Latex grammar chosen.  

Can I force TextMate 2 to use Markdown for all .txt files by default while allowing me to selectively change some .txt files to other grammars, without changing the default grammar for all files of that type?  

Thanks!!