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Hi, I don't believe that's feasible. For txt files you only have the option of 'Simple' or 'Office files' or some such name in File Metadata. None of the available fields are other than text.
Not custom columns in explorer as far as I know, but you can, of course, experiment with Custom Profile in File Metadata's File Association Manager. As to whether attempting to apply fields not normally associated with a given file type is meaningful- well, that's another question.
By the way, you will probably have noted the correlation between available column headers and fields available in the Properties, Details sheet for a given file type, and similarly those in the Details Pane in f. explorer.
Totally out of the box Idea ... although the addition of actual images are not supported, there are Images that are characters in the full Code Page used for the full selection of acceptable characters in Windows. these sets are available and you can use this to add various "Pictograms"
There is also a method of holding down ALT + xxx or 0xxx to insert the pictogram some may be suitable for your purpose, a few examples available using the ALT method, or the Character Map applet ...
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These work here in the forum and I have tested them in the Metadata for Audio Files so hopefully will work with your setup, (these are MS arial by the way so nothing exotic)