Excel 2010 Extension Warning On Opening Excel Workbook from a Web Site


I’m having a hard time understand the difference between Consistent Mime Handling and Mime Sniffing Safety feature.  The original is that I have a web app which users can login and retrieve reports in an Excel format but in the background it actual is an html file.  The users are getting an
Error every time they run a report “The file you are trying to open, '[filename]', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"  (Yes | No | Help)”
Users are running Windows 7 Sp1 and Office 2010.  Which of these two policies will force the file to be save and open in an xls format.

Thank you in advance
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From http://technet.microsoft.com/en-us/library/bb457144.aspx:

Consistent MIME Handling
Internet Explorer uses Multipurpose Internet Mail Extensions (MIME) data to determine file handling procedures for files that are received through a Web server. This policy setting determines whether Internet Explorer requires that all file-type information provided by Web servers be consistent. For example, if the MIME type of a file is text/plain but the MIME sniff indicates that the file is actually an executable file, Internet Explorer renames the file by saving it in the Internet Explorer cache and changing its extension.

MIME Sniffing Safety Feature
This policy setting determines whether Internet Explorer MIME sniffing prevents promotion of a file of one type to a more dangerous file type. For example, it does not allow script to run from a file marked as text.

Also, for an Excel-centric view on this message, see http://support.microsoft.com/kb/948615


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