Content type for excel files

The content type for .xlsx files is:

application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

Or use 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/application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";

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

Or if you are trying to read the file then try this:

DataSet objds = new DataSet();
string ConnStr = "";
if (FileExtension == ".xlsx")
{
    ConnStr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + FileName + ";Extended Properties="Excel 12.0 Xml;HDR=No;IMEX=1";";
}
else
{
    ConnStr = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + FileName + ";Extended Properties="Excel 8.0;HDR=No;IMEX=1";";

}
OleDbCommand selectCommand = new OleDbCommand();
OleDbConnection connection = new OleDbConnection();
OleDbDataAdapter adapter = new OleDbDataAdapter();
connection.ConnectionString = ConnStr;
string strSQL = "SELECT * FROM [Sheet1$]";
if (connection.State != ConnectionState.Open)
    connection.Open();
OleDbCommand cmd = new OleDbCommand(strSQL, connection);
OleDbDataAdapter da = new OleDbDataAdapter(cmd);
da.Fill(objds);
connection.Close();

All the best.
—Amit

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