It's possible that your installed browser might lack support for those fortan files. There are few file formats that can be opened inside a browser, and because of that you encounter the issues. Simply right click the fortran files and if the MED application supports them, then they should be displayed.
Right click a file, and select Open With. From the list that will open, choose MED Texteditor or browse to the file and select the executable, then every time you double click a fortran file, it will be opened with MED Textedit.