Not all file types will be displayed on the Template Editor except for:
.rtf
.htm
.txt
.doc
but clicking on Launch External Editor, will open the file in its original application, i.e. as a spreadsheet or PowerPoint presentation etc.
It can be edited there.
Save any edits when you close the application.
and save again when you close the Template Editor.
Please ensure that the user has access to the required program(s).