Hi,
the document extension is stored under the document version, so you have two possibilities:
1) Use the DOCU endpoint and fetch the document versions by querying the nested field versions:ext
https://veridiancu.my.workfront.com/attask/api/v18.0/docu/search?id=6658f66f004a4……………..&s…
or
2) use the document versions endpoint DOCV
https://veridiancu.my.workfront.com/attask/api/v18.0/docv/search?documentID=6658f66f004a4…………..
Regards
Lars