I have SL5500 with Sharp Rom 3.13 and I can tell you from experience that the stock file manager and text editor are, to say the least, JUNK.
This file manager will only open text files with the stock editor, and they have to have a .txt extension, or they don't save right, plus some files don't show the text at all.
I use Tree!Explorer Plus file manager. It comes with its own text editor, plus you can "send to" any editor or other program you wish (configurable).
I couldn't get along without it.
Doug