I may have the wrong idea here, but I thought that:
MIME types are identifying codes, embedded inside a file.
File .extensions are idenifying codes, suffixed to the file's name.
I thought, from something I heard in the dim dark ages, that Linux was .extension agnostic... ( maybe it was in the early days, and things have changed since then...? )
I've recently come from the Windows world, where, at the Operating-System level, a file .extension is the only way (as far as I know) to associate it with its relevant Application program.
Because I don't know why, I find it a bit disconcerting that a file named "fred" and a file named "fred.txt" both open up in a Text Editor.
Is there a clear-cut hierarchy at work here?