I recently had to replace a workstation (the one I actually use dtd) 

One thing I didnt do straight away was get the External Edit setup.  

So when I eventually realized I needed it I downloaded OpenWebstart and ran it.  

Problem: the file would download and I could run it once with openwebstart and it would open Libreoffice as a webdav link and everything but I had to associate the file with OWS everytime.  

Reason: The download did not have a file type extension .  It was simply called 

"externaledit"

I opened my old terminal downloaded the same link.  Yep it downloaded as "externaledit.jnlp" 

I looked at the download link it was exactly the same.  

The difference was on the old machine I had JAVA  as  a JRE installed - the new one didnt.  Yes I had JDKs installed but not the oracle JRE 1.8.  

As soon as I installed it - the files downloaded with the correct extension. 

For now unless someone else has a better solution I think you will need the oracle JRE on workstations to ensure the file has the correct association.