A file must contain the correct suffix (e.g. CBL, JCL) in order to be opened with the required editor.
The assignment of an SCLM member to a file suffix typically depends on its assigned SCLM language. In some cases, the SCLM type alone can also be enough to make this assignment possible.
You can use the exit TAUSCXMS to associate the values of the file suffix to the SCLM member.
As long as no specific editor exists for the SCLM member, TXT should be assigned as a suffix.