In order to open .tcl extension files in the Editor window, a File Association needs to be created. To create a File Association, follow steps below:
The Preferences dialog box appears.
The File Associations preferences appear on the right hand side.
The File Associations pane appears.
The New File Type dialog box appears.
The .tcl extension appears under File Types .
The Editor Selection dialog box appears.
The .tcl extension file types get associated with a Text Editor (as the following figure shows).
You can now open the tcl extension files in the Editor window.