I tried: "[ ] Use a copy (will be stacked with original)" (guessing, having found no documentation on what this is supposed to do) but that gives only:
All of the Locate and Send To commands are designed to operate
on files, not on directories, which is causing your trouble. When you right-click on a Folder in the Disk Location views, it is trying to send the entire folder
and all of the files it contains to the selected external application, not just the folder itself.
So, for example, if you use [Filename], and right-click on a folder full of files, it'll send each file in the folder (one at a time) to the external application. If it is a big folder, it'll probably make your computer run out of RAM.
The use a copy (stacked) option, to explain, creates a copy of the file before sending it to your external application. So, the idea would be, you right-click on an Image File and choose Send To (external) > Photoshop. MC will then create a duplicate of the image file (preserving the original) and send the duplicate to Photoshop, where you can make edits. Then this edited version will be in a file stack with the original, so that you can access both the edit and the original in the same place in MC automatically.
The problem with the [Filename (path)] field is that it is removing the "filename" part of the path, which in this case, since it isn't there, ends up being the selected folder name. That would have been a good trick, otherwise, but it doesn't look like it works.