Hello.. I'm no expert and I've not done this particular edit, but I can say that there is some misunderstanding here. "Open in explorer" is actually "Show in explorer" and simply means to be in TRS19 Content Manager but to "show" the file in Microsoft Explorer (WIN 10) or in the Finder (Mac OSX). (ie: you are in TRS19 Content Manager and you right click the asset, select "Open" option, then "Show in Explorer". Once you are viewing the file on your computer in Explorer or the Mac Finder you need an app to open the file type you are interested in. For a "txt" file, with Win 10 that could be Notepad, with Mac OSX you would use TextEdit. A .txt file is simply a plain text file. To open any type of image file you would need to be seeing the file in Explorer or Finder (OSX) and then open the image with what ever image app is suitable for that type of file, such as Images2TGA, or whatever app you plan on using to do whatever it is you are doing.
Explanation of what's happening when you do the above:
From Content Manager, when you "Open/Show" in Explorer or Finder (Mac) you are basically opening a compressed file into its components. Thus, when you are done editing the text or image file you need go back to in Content Manager and "Submit Edits." This packs everything up into a suitable TRS19 file again. If you open assets from Content Manger and leave them open they will not work in game. You must leave the files where they show when you "Open" them in CM.
I hope this helps. It just like having a zip file package, un-zipping and working on one file, then zipping the package back again.