I have a clickonce application with a custom (xml based) file association. Users can dbl-click a file from the file system and it will launch the associated clickonce app.
On Windows XP, users can attach the file to an Outlook (2007, SP2) message and dbl-click the attachment to launch the app as well.
However, in testing for an upgrade to Windows 7 (x64), the clickonce app will launch correctly when the file is dbl-clicked from the desktop, but when attempting to open the file from an email attachment, outlook displays the "Opening Mail Attachment" dialog but does not launch the clickonce app. If I save the attachment to the desktop and dbl-click it from there, it does launch the clickonce app as expected.
This might be relevant, I noted that the outlook attachment displays the clickonce application's icon when running on WinXP, but displays a generic icon on Win7. The application's icon is displayed when the file is saved to the desktop on both WinXP and Win7.
(displays correct icon and launches clickonce app)
FileSystem Outlook Attachment
Win XP Yes Yes
Win 7 Yes No
The registry entry for the file type under HK_CLASSES_ROOT is the same on both systems with the exception of the Guid key having different values.
I examined the event logs on the Win7 machines, but did not find anything that appears related.