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A Web server that uses the Content-disposition: attachment HTTP header to force a file download should prompt the user to open or save the file; however, in Internet Explorer 5.5 Service Pack 1 (SP1), the user is prompted to open or save the HTML file instead.
To resolve this problem, refer to the hotfix that is described in the following Microsoft Knowledge Base article:
281119 Internet Explorer Saves HTML Content Instead of the Active Document
In addition, a fix for this problem is incorporated in Microsoft security bulletin MS01-015: