Copy the following to /applications/copy-ext.app
, run it from the Applications tab, and it will copy the file for you to the SD card.
cp /ebrmain/config/extensions.cfg /mnt/ext2/
If you use Windows, use an editor like Notepad++ to write the file and only put linefeeds at the end of the lines (and not carriage returns). Otherwise, it won't run the script.