Прочитать файл Excel из потока

Это легко сделать с помощью c-sharp EPPlus.

//the excel sheet as byte array (as example from a FileUpload Control)
byte[] bin = FileUpload1.FileBytes;

//gen the byte array into the memorystream
using (MemoryStream ms = new MemoryStream(bin))
using (ExcelPackage package = new ExcelPackage(ms))
{
    //get the first sheet from the excel file
    ExcelWorksheet sheet = package.Workbook.Worksheets[1];

    //loop all rows in the sheet
    for (int i = sheet.Dimension.Start.Row; i <= sheet.Dimension.End.Row; i++)
    {
        //loop all columns in a row
        for (int j = sheet.Dimension.Start.Column; j <= sheet.Dimension.End.Column; j++)
        {
            //do something with the current cell value
            string currentCellValue = sheet.Cells[i, j].Value.ToString();
        }
    }
}