How to create file type association for web cache files

How can I create a file type association for web cache files such as from Firefox which have no file extension?

For example, images in the Firefox cache have file names which are named like this "01D413A2d01".

I want (need) to set up a file association of "." or ". " and link (group) this with other image file type associations. I tried adding these to the images group in file types in the settings menu but couldn't get it to work, probably because I didn't know all the required information. e.g. Do I need to enter MIME information and if so, what do I put in, etc?

Many thanks in advance.

You can't. That aspect of the file type system is based on file extensions.

Most of Opus's viewers, thumbnailers, etc. will still work, however, as they look at the file contents and not the file extensions.

(There is a special "(NONE)" extension but you would not want to use that for anything like this as there are lots of files that have no extension and are not images, even within the Firefox cache folder.)

Many thanks. I can see now that choosing 'images' works just right.