Files invisible to NP++..?

Background: The Win7 StickyNote application has crappy fonts, but on the web, I found a suggestion to use NotePad++ to edit a config file of some sort, (an .mui file. It was suggested that Notepad might corrupt the file, because it has binary components, but NP++ should handle it.)

The file is:
"C:\Windows\System32\en-US\StikyNot.exe.mui"

When I look for the file, NP++ does not have the file in its File-Open list. Several other .MUI file types are missing. Notepad opens this file fine, but why is this file not showing on my NP++ File Open menu? Does it show on yours? I have sort of lost interest in the StickyNote problem, but would really like to know why I cant even see this (and some other .mui) file in NP++.

If you mentioned which version of NP++ you were using, I missed it.

I don't use NP++ - but I installed it from here...
Notepad++ v6.0
...for this test/post.

Maybe I opened the file in a different way than you did.
I used File > Open and worked my way to the file.

Thanks for the comeback UNI.
Was using NP++ ver 5.9.6.2. I posted on the NP++ forum, but there is not much traffic there.
Further investigation, Another computer with an earlier version of NP++, the files are visible just fine. I will try ver 6 later and post results back. Was really disconcerting though. Looks like the program uses a Windows Explorer standard frame to navigate to the file-open area. And I did not realize that there was much flexibility in what Explorer shows.