Hi Kenneth,
.NET Framwork 3.5 supports file associations in a ClickOnce application.
The requirements are as follows:
1) File associations are only supported for Orcas apps and up (look for an
assembly dependency on system.core.dll or greater in the manifest)
2) Your app must request Full Trust Permissions and be Offline Capable
(deployment generates shortcuts/ARP entry)
3) Only stand-alone apps are supported. File associations are not supported
for WPF browser hosted apps or VSTO plugins
For each file association you want your app to support, add xml similar to
the following to the application manifest (.exe.manifest)
<fileAssociation xmlns="urn:schemas-microsoft-com:clickonce.v1"
extension=".nyt"
description="New York Times Document"
progid="NYT.Document"
defaultIcon="nyt.ico"
/>
Currently, we can't either set file associations from the Properties
Designer of a project or use Shell Set File Assocations UI to manage
ClickOnce file associations. We can only add the above xml to the
application manifest manually. We are working with the Shell team to
provide better support for this in the future.
For more information about ClickOnce file associations, please refer to the
following MSDN document:
http://msdn2.microsoft.com/en-us/library/bb882599(VS.90).aspx
Hope this helps.
If you have any question, please feel free to let me know.
Sincerely,
Linda Liu
Microsoft Online Community Support