Trouble importing dashboard

Hi,

following on from Trouble with dashboard import in ECE cluster in the ECE forum, where I described being unable to import a dashboard to Kibana.

I'm using a command very similar to the one in the documentation to try and import a dashboard from here:

I know the dashboard was developed with Kibana 6.x and my ECE cluster is running 7.x

The curl command is like:
curl -v -X POST -u elastic: https:///api/saved_objects/_import?overwrite=true-H 'kbn-xsrf: true' --form file=@file.ndjson

The curl transfer seems to hang after headers around the point where it gets HTTP 100 Continue so I'm not sure Kibana is seeing the content or failing to process it. Not really getting that far. Please check the post linked above for the curl output...

Any suggestions for debugging this or manual approaches to upgrade the dashboard and import it?