I finally got smart (I think).
Since there is no program available that I know of to undelete files, I created a directory on my SD Card called "Recycle Bin".
With the help of Teee!Explorer QT Plus+, I move files that I want to delete to the "Recycle Bin" for safe keeping, just in case I decide I shouldn't have deleted them.
Later, I can delete them if I decide it is safe to do so.
It would be great if there was an "undelete" type of program available, but so far I haven't seen one.

I am running SL-5500 with Sharp Rom 3.13.