To change filetype associations in Gnome, right-click a file of the type whose associations you want to change and choose 'Properties'.

To change the default icon for that filetype, click the icon. Then use the Gnome common file-open dialog to locate the icon you would like to use.

To change the default viewer for that filetype, click the 'Open With' tab, and select the application with which you would like to open files of this type by default. To add an application to this list, click the 'Add' button. The 'Add Application' dialog will give you a larger list of applications to choose from. If you still do not see the application with which you would like to open files of this filetype by default, choose 'Use a custom command'. and type the path to the default viewer of your choice (e.g.: /usr/bin/vlc) or use the 'Browse' button to choose that application with the Gnome common file-open dialog.

Thanks for providing info. Next time though maybe use a title that makes this visible as a kind of answer or tip. (And still the practice of using only a file extension mapping to determine how to handle a certain file strikes me as odd as say Firefox parsing Javascript code in what seems to be presented as a JPEG (which IIRC was used in a recent PoC)).

...<scratches head>.... I don't see where I ever mentioned a file extension. GNU uses the magic bytes (first two or four bytes of a file) to determine file type if possible.

Yeah, you're right. I was thinking of another filemanager.