How to determine real format of an XLSX Excel file?

I have a well known problem that is described in Extension Warning On Opening Excel Workbook from a Web Site microsoft blog entry. I've added URL rewrite to have URL nicely formatted and my mime type matches exactly XLSX recommended file type. However I still get a warning. I suspect tha开发者_开发问答t service that provides me those xlsx files mismatches real file format and extension.

Is there a way to determine real xlsx file format? Something that would say what is the native extension for particular Excel file.

Thanks in advance.

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Have you tried changing the mime header from vnd:excel to octet-stream? This will still bring Excel up, albeit not embedded into IE, which vnd:excel does (but I hate vnd:excel anyways because embedding the spreadsheet into the browser screws up the form flow of my web apps).

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Did not find an answer for that anyway. However I've discovered the reason why I get a warning from Excel - any parameter in query string will trigger such a warning, even for static files:

http://localhost/1.xls

works ok

http://localhost/1.xls?testparam=paramvalue

gives a warning.

Will use URL rewrite to encode parameters.