After upgrading MOSS 2007 content databases to SP2010, an InfoPath form template (template.xsn) that in the past could be either opened or saved was now only able to be saved, not opened.
Comparing Fiddler2 traces of the traffic for the .xsn file on the MOSS2007 and SP2010 farms showed that the key difference in the headers was this: “X-Download-Options: noopen.” Here's a great reference for that. SharePoint 2010 and 2013 are a bit more strict here than MOSS 2007 was.
Since the problematic header also showed clearly that that the content type was "application/octet-stream" we added the necessary MIME type with the following PowerShell commands:
$webApplication = Get-SPWebApplication "http:/yourwebapplicationurl" $webApplication.AllowedInlineDownloadedMimeTypes.Add("application/octet-stream") $webApplication.Update()
Do you want to save this file?