Cannot install Jira Watcher plugin via UPM - JIRA 5.1.8

[aarnetadmin]

Getting the following when installing the Jira Watcher plugin from UPM:

Could not find a handler capable of installing the add-on at https://marketplace.atlassian.com/download/plugins/com.burningcode.jira.issue.customfields.impl.jira-watcher-field/version/26. Check that the file is a valid add-on.

I've double checked to make sure the plugin is a valid jar file for a plugin - it seems to check out ok. Not sure what to check next.

[RambanamP]

you can submit support request here by updating error messages

https://bitbucket.org/rbarham/jira-watcher-field-plugin/issues

or check the contact details

https://marketplace.atlassian.com/plugins/com.burningcode.jira.issue.customfields.impl.jira-watcher-field/version/86/support

[aarnetadmin]

And ... by placing the jar in the installed-plugins dir it worked. I officially have no idea what is going on!

[aarnetadmin]

UPM version is 2.13.2.