Does anybody know how to find the "external key" of files in Content Builder? I'm trying to add an attachment to an email using the AttachFile AMPScript.  The script instructions state that Content Builder hosted files need to be called in the script using the external key.  None of my content builder files have external keys.  I only see "Customer Key" "ID" and "URL" for my files.

For reference, the AMPScript instructions I'm following are here https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-programmatic-content.meta/mc-programmatic-content/attachfile.htm?search_text=Email%20Attachments. 

Thanks