Following mime types should be registered in IIS, when using VSTO and ClickOnce:
.application
application/x-ms-application
.manifest
application/x-ms-manifest
.deploy
application/octet-stream
.deploy
application/x-ms-vsto
Read the complete post at http://developers.de/blogs/damir_dobric/archive/2008/07/22/mimetypes-required-for-vsto-and-clickonce.aspx