This morning I ran into the issue when I went to install a ClickOnce application from the publish.htm page it was opening up the .application file in notepad. I figured this had to be the MIME types so into the registry I go. Sure enough the entry for .application was slightly off. If you have this issue go and check:
HKCR\.application
You should see:
Name: (Default) Data: Application.Manifest
Name: ContentType Data: application/x-ms-application
After confirming these settings I got onto another error “The system detected a possible attempt to compromise security. Please ensure that you can contact the server that authenticated you.”. This error was caused by not being authenticated to download from the server. There are a couple solutions to this.
1) On the server allow anonymous access to the virtual directory. Be sure to set up permissions on the folder as well. More info can be here.
3) Require user to be authenticated when hitting the site.
Depending on your situation you may want each person to be authenticated. This is the case I’m in so I chose to use option two as the smartclient should only be available to authenticated internal people.