I cannot open some text files with an embedded text editor depending on the extension in the file name.
For example, if I have Wavefront .obj file, KDevelop will open it with hex editor, in the Open With menu only hex editors are available.
There is File Type Options button in the file properties, but if I click to this button nothing happens and no output in stdout/stderr from KDevelop.
I am using Gentoo Linux distribution with KDevelop 5.5.2
Had same problem, and here's quick solution:
KDevelop after that, stops use hex editor and open files in embedded text viewer. Note, that this recipe would work only for one file type (or extension), that was selected in first step.