Hi alhamd,

Refer below code.

HTML

<asp:FileUpload ID="fuUpload" runat="server" AllowMultiple="true" />

Namespaces 

using System.IO;
using Microsoft.Office.Interop.Outlook;

Code

protected void SendMail()
{
    MailItem oEmail = (MailItem)new Application().CreateItem(OlItemType.olMailItem);
    oEmail.Recipients.Add("email@outlook.com");
    oEmail.CC = "cc@outlook.com";
    oEmail.Recipients.ResolveAll();
    oEmail.Subject = "Your Subject";
    oEmail.BodyFormat = OlBodyFormat.olFormatHTML;
    oEmail.Body = "Email Body";
    oEmail.Importance = OlImportance.olImportanceHigh;
    oEmail.ReadReceiptRequested = true;

    foreach (HttpPostedFile file in fuUpload.PostedFiles)
    {
        file.SaveAs(Server.MapPath("~/Files") + Path.GetFileName(file.FileName));
        oEmail.Attachments.Add(Server.MapPath("~/Files") + Path.GetFileName(file.FileName), OlAttachmentType.olByValue);
    }

    oEmail.Recipients.ResolveAll();
    oEmail.Save();
    oEmail.Display(false);
    oEmail.Send();
}