Big deal?
Change your code like below(Just need to concatenate the datetime to filename)
try
{
string strCurrentDateTime = DateTime.Now.ToString("dd-MM-yyyy hh.mm");
rpt.ExportToPdf(Server.MapPath("reports\\") + ASPxDropDownEdit1.SelectedItem.Text + "-ConsumableReport" + strCurrentDateTime + ".pdf");
}
catch (Exception ex)
{
}
Attachment attach = new Attachment(Server.MapPath("~\\reports") + "\\" +ASPxDropDownEdit1.SelectedItem.Text + "-ConsumableReport" + strCurrentDateTime + ".pdf");