Well after searching a bunch, it looks like OWA on Exchange 2010 is broken on windows 10's edge browser. It opens fine, but won't open the attachment window. Fine job there MS!
There are 3 workarounds, use a different browser, open in IE via edge, or run the light version. We all know how users are, and all options kind of suck in their own way. I think I've settled on the "best" crappy way is to use the Open in IE option, but now how to convey that to Joe user logging in from his shiny new PC at home for the first time.
Ideally what I want to do is just Add a big red text line to our logon page. Something that says.
Windows 10 users, Microsoft Edge does not support OWA. Please open in IE by clicking the 3 dots in the top right, and choose Open in Internet Explorer.
How would I go about doing that. I've read a bit about the OWA themes, but nothing I've seen addresses adding a paragraph to the logon.aspx page.