Safari extension won't install

[tom....@gmail.com]

When I open markdown-here.safariextz it shows this warning:

Are you sure you want to install the extension “Markdown Here”?
This extension is not from the Safari Extensions Gallery. You should only install extensions from sources you trust.
Cancel / Trust

but when I click Trust it fails to install

My Safari version is Version 11.1 (13605.1.33.1.4) on High Sierra

[tom....@gmail.com]

On Tuesday, 29 May 2018 12:50:41 UTC+1, tom....@gmail.com wrote:
> but when I click Trust it fails to install

OK I finally worked out why it doesn't install for me: I don't have admin privileges, but surely I'm not alone in that? I managed to install it on safari beta where it prompted for admin authorisation which seems like the correct behaviour so maybe the fix will come in safari 11.2?