The problem is that in Linux, the format determines the nature of the file
and the suffix is largely for convenience. You can have the same content in
myfile and myfile.txt. The txt suffix does nothing. Because of scripting in
Linux text files do not open up as you are used to. If the text file is a
script then you may want to edit it or run it as an executable.
The solution is to stop thinking like a Windows user and retrain yourself.
Right click on the file and then choose open with, gedit.