He only needs the relatively small "GarminDevice.xml" file which you'll find in the device's Garmin folder. Copy it then remove your device's UID, look for this:
"</Model>
<Id>1234567890</Id>" as example and remove the ten numerals in your file. You'll have to then compress it as a ZIP or RAR as XML files are not allowed for upload to our server.

To attach it on your existing post use "Edit" button. For a new post you first need to go to advanced editor by using "Go Advanced" button. Then scroll down to "Manage Attachments" button.