It seems that the control does not open an RTF extension with plain text inside it. You can try to change the extension of a text file from TXT to RTF. You could not open it, the control remains empty, even if you can open it in other apps.
The openfile/savefile routines are not working fine. It is not with the control itself.
With PlainText set to False, the plain text is not shown, no matter it has an extension TXT or RTF.