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