How to make Windows Explorer treat a file with a `.MAS` extension as a regular file

I have a regular file on a Microsoft Windows 7 computer called MYFILE.MAS, as viewed from (e.g.) cmd.exe. (You can create one too, it's just an empty file). However, when viewing this file graphically with Windows Explorer, this file is treated differently than any other (e.g. to MYFILE.XYZ):

(My view options should always show file extensions). Viewing the properties and attempting to use the right-click menu is different than any other regular file:

I can't even rename the file in Windows Explorer to another extension (this needs to be done in a shell, i.e. mv MYFILE.MAS MYFILE.MAS1).

Is there a way to make Windows Explorer to treat a .MAS file extension as a regular file? I can't find any Microsoft documentation to suggest why this is happening. An no, I'm not doing anything related to Microsoft Access.

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