How to: Save a file with custom 3-character extension.
Solution:
Select 'Save As' and select the file type. Then type the filename and extension enclosed in quotation marks.
1) Select the 'File' menu and select 'Save As...'.
2) Select the file type from the 'Save As Type' drop-down list.
3) Type the filename in quotation marks in the 'File Name' box.
EXAMPLE: ''Project.rmg''
4) Click 'Save'.
