SUMMARY Opening .m files should open instead with Octave/Matlab support; the results for ".m file extension" in a Internet search engine far outnumbers those related to Objective-C (whatever Objective-C is).
Searched on GitHub for filename:".m" Unfortunately this search matches "m" anywhere in a filename. Still there are 11,754,262 MATLAB results and twice as many - 23,123,935 - Objective-C results. You can change the priorities of these file types on your own system according to what you use more often in KWrite/Kate/KDevelop Settings=>Open/Save=>Modes & Filetypes.
I think Objective-C is a lot more common than Octave/Matlab for many people. I naturally see that this is not convenient for you, but there is the workaround to alter the priority.