I know this sounds silly, but try making the url end with .csv I wrote a perl cgi script some time ago that spit out a csv file. When it ended with .csv, IE seemed to ignore the server's Content-type: text/html header.
http://finance.yahoo.com/q?s=WCG&d=v1 Look, my stock is falling! Actually, if you look at the link below and to the right of the table, you'll see a download spreadsheet link. Yahoo uses a &e=.csv to set the extension. Presumably, the e value is for extension.
That isn't silly, it corresponds exactly with the description you can find at http://msdn.microsoft.com/workshop/networking/moniker/overview/appendix_a.as... There term silly seems much more suitable for that, btw. Btw., it shows that changing the extension to .csv won't help until one has registered an application for .csv-files in windows. And in fact, the "spreadsheet"-link showed up in my IE, no download window came up... cheers, oliver