How to remove array items that include a certain substring?

I have an array list of filenames that I want to process—let's say some of them start with the letter "a", some start with "b", and some start with "c'". I only want to filter out all of the array items that start with "a" and "b" so the array only includes the "c" filenames.

How could I do this?

%creating sample array
filetable = {'a1','a2','b1','b2','c1','c2'};