Change Individual File Type

In my school work I have a lot of MATLAB .m files with which I have to work. I would like to be able to open all of them in Kate, but I have different file types for the same file extension and would like to change them.

Some of the .m files are already text/x-matlab files and open in Kate and that's great, but some of them are text/x-tex files and open in Kile since that is my default for Kile, and some are text/x-objcsrc files and they also open in Kate. If I change the default for the x-tex files to Kate then all of my actual .tex files will no longer open in Kile and I don't want that. What I'm trying to do is change the file type of the individual file so that what is right now a text/x-tex file will become a text/x-matlab file.

Every search I've done so far has shown me how to change the default application which I don't really care about as I've already done that.