I tried all the options but it's impossible to add it from Windows 8.1 x64
You can change the file type association with any program but when you try with "protocols" (that are in the low end of the same big list under Control Panel) the system will only let you pick on his default list or "apps" (looking in the store), not regular desktop programs.
I tryied uninstall/reinstall without luck. Then I remembered that this thing has already happened in the past, and some gentle soul here on the forums has given me this .reg file to add:
Windows Registry Editor Version 5.00
@="C:\Program Files (x86)\Opera Mail\operamail.exe,0"
@=""C:\Program Files (x86)\Opera Mail\operamail.exe" "%1""
So I risked and blindly added this thing. The only result was removing every program in the short "default" list that is in Control Panel default association types.
Anyone can can help? I'm at a dead end :|