Since XnViewMP 0.50 (Windows) we have the resurrected ability to associate file-types to XnViewMP using it''s supplied 'Associate' tool.
There are some minor suggestions to functionality which I have noticed needs improving:
1.) After associating my file-types with XnViewMP, I used some file-type related tools such as File-managers which can analyze and display the file-extensions. I observed that some extensions need to be considered in a future version.
screenshots of my detection (click to enlarge):


rle <-----in future must use same XnviewMP file ext icon as BMP
jfif <-----in future must use same XnviewMP file ext icon as JPG
jpe <-----in future must use same XnviewMP file ext icon as JPG
nef <-----in future must use same XnviewMP file ext icon as RAW
references:
http://www.xnview.com/en/formats.html
http://www.why-not.com/articles/formats.htm#BMPDIBRLE

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2.) Small suggestion: The XnViewMP 0.53 association tool currently does not write "Type Name" to each file Extension in the registry. A quick use of the freeware tool, FileTypesMan, shows this to me.
for example;
For Bitmap (.BMP), XnViewMP currently just describes it as "BMP file (.bmp)" - When I think it could be better described as "XnView Bitmap (.bmp)"
from what I understand the Win Registry has two fields to be use which are 'type name' and 'type description'
two screenshots which show different associated file extension on my system giving a suitable 'description'; ========================================================================
3.) Right, according to what FileTypesMan is telling me on my system, it seems that XnView 1.9x standard supports ICO association without changing file-type icon for .ICO . This is really impressive that XnView 1.9x does this and is the native way Win Explorer does it too.
Maybe what I did was to manually associate .ICO with XnView 1.9x using Win Explorer's "open with" <--'set default application' , which is what could have made Win Explorer assign no icon to it and leave it default because remember .ICO on Windows does not need file-type icon in association because it is an 'icon' itself.

I would like XnViewMP to do it this same way too.
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4.) Question: Where is the 'ICNS' file extension written to in registry? XnViewMP has associated it on my system but I cannot find it in FileTypesMan...