Perhaps, after modding , you need to call it xxx.asm and also while saving you need to define as text only file
Of course, i use word pad for that.
the principle is same even if you use open office. that while saving you need to define the file as text only. then i after opening this file in MPLAB, and if you dont find the style of .asm file with colors and all, i generally cause a small mod like adding a space, to enable me save it. now i save it as .asm file
and reopen, presto, i find the feature that are expected.