Hello Mathias.
A nice idea to improve MC.
If I can put forward my thoughts?
The 'Heading Bar' at the top of the list display with the options for "Name","Ext","Size" and "Date" is counter-intuitive to the format of display we are expecting in a list display.
That is to say if I select 'List' and my files are then arranged in columns displaying my files by "Name"(like your screenshot) then the 'Heading Bar' should not look like it is Heading columns for undisplayed information such as "Size" or "Date". Rather the 'Heading Bar' should be removed and the options to change the style of sorting the files should come from a small dropdown box, like the Drive box or maybe an option in the right-click menu, personally I prefer the right-click option.
Also I totally agree with Jungle about the options with the extensions being configurable. Show them, don't show them, right-aligned, natural, the more ways there are to display files then MultCommander is living up to its name.
I can't wait to see what results you bring.