How to open Excel file stored in byte[] via OleDB?

If you want to read using OleDB, then you have to write the bytes to disk. For example, you could do this:

var filename = System.IO.Path.GetTempFileName();

// Assuming that fileBytes is a byte[] containing what you read from your database        
System.IO.File.WriteAllBytes(filename, fileBytes);
var connection = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + filename + ";Extended Properties=\"Excel 12.0;HDR=YES\"";

// Do your work on excel
using (System.Data.OleDb.OleDbConnection conn = new System.Data.OleDb.OleDbConnection(connection))
{
    conn.Open();
    using (var cmd = conn.CreateCommand())
    {
        cmd.CommandText = "SELECT * FROM [Sheet1$]";

        using (var rdr = cmd.ExecuteReader())
        {
            while (rdr.Read())
            {
                System.Diagnostics.Debug.WriteLine(rdr["ColumnName"]);
            }
        }
    }
    conn.Close();
}

//Cleanup
System.IO.File.Delete(filename);

If you don't want to write the file to disk, you could look into using a third party library that can read excel files from a memory stream. Tools like SpreadsheetGear or Aspose are commercial tools that can accomplish this.

I don't know anything about what library you're using to deal with your Excel data, but the first thing that comes to mind is that it almost certainly has a LoadFromFile method. It might not be called that, but that's what it does. See if it also has a LoadFromStream method, and if so, take your byte[] data and load it into a MemoryStream, and load the XLS from there.

emp.upload_file = Path.GetFileName(file.FileName);

emp as your table object created and file is HttpPostedFileBase file