Can Gedit default highlighting style be set for files without an extension?

Steps taken

In my gedit, from the top bar menu, I select View, Highlight mode... and "Plain Text" is the default as shown below:

Using the file name "test.sample" and pasting in a few lines of Matlab code it still stays as "Plain Text" format unless I force it to "Matlab" format.

After saving and exiting I performed cp test.sample test.newbee followed by gedit test.newbee the default format is still "Plain Text".

Next I created a new file using gedit called noextention. I typed a sentence, saved the new file and exited. Then I reopened the file using gedit and this is the result:

Notice the bottom bar of gedit's window shows Plain Text option for highlighting.

Summary

This was done under Ubuntu 16.04, Kernel 4.4.0-53.

I can confirm that <property name="globs">*.m</property> exists within /usr/share/gtksourceview-3.0/language-specs/matlab.lang.

Because I can't break my system like yours, unless we find something on your system to fix, I suggest reinstalling gedit after removing and purging all it's files with apt-get.

Particulars about my version:

$ sudo apt install gedit
Reading package lists... Done
Building dependency tree       
Reading state information... Done
gedit is already the newest version (3.18.3-0ubuntu4).
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.