MIME-Type for Excel XML (ASP.NET 3.5)

Use like this

Response.ContentType = "application/vnd.ms-excel";

Response.AppendHeader("content-disposition", "attachment; filename=myfile.xls");

For Excel 2007 and above the MIME type differs

Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";

Response.AppendHeader("content-disposition", "attachment; filename=myfile.xlsx");

See list of MIME types

Office 2007 File Format MIME Types

EDIT:

If the content is not a native Excel file format, but is instead a text based format (such as CSV, TXT, XML), then the web site can add the following HTTP header to their GET response to tell IE to use an alternate name, and in the name you can set the extension to the right content type:

Response.AddHeader "Content-Disposition", "Attachment;Filename=myfile.csv"

For more details see this link