[itsme]

I removed all predefined "Source Types" entries and created my own source type list containing something like "layout info".
txt, pro, LS (leadsheet) and the like are entries there. Thus I can easily filter for txt files or use the "Source Types" tab to select certain types of scores.
Additionally I added "Source Types" to the caption (Settings > Library Settings > Song Title Formatting). As soon as "Show Caption" is set all the lists display a second subtitle line for every song showing additional info like source type or whatever you prefer to configure.

I use "Collections" to sort my songs by band / line-up.