How to send an email with attachment and connect it to a system entity, e. g. an Incident

using Microsoft.Crm.Sdk.Messages;
using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Tooling.Connector;
using System;
using System.Configuration;
using System.IO;
using Xrm;

namespace CrmDevelopersHowTo
{
    public static class SendEmailSamples
    {
        public static void SendEmailWithAttachment()
        {
            // Initiate CRM-connection
            var conn = new CrmServiceClient(ConfigurationManager.ConnectionStrings["CONN"].ConnectionString);
            var service = (IOrganizationService)conn.OrganizationWebProxyClient != null ? (IOrganizationService)conn.OrganizationWebProxyClient : (IOrganizationService)conn.OrganizationServiceProxy;

            Guid regardingObjectGuid = new Guid("----- Incident Id -----"); //Example - could be a lot of other entity types
            EntityReference regardingObjectRef = new EntityReference("incident", regardingObjectGuid);

            Guid senderGuid = new Guid("----- System User Id -----");
            EntityReference senderRef = new EntityReference("systemuser", senderGuid);

            ActivityParty fromParty = new ActivityParty()
            {
                PartyId = senderRef
            };

            Guid recipientGuid = new Guid("----- Contact Id -----");
            EntityReference recipientRef = new EntityReference("contact", recipientGuid);

            ActivityParty toParty = new ActivityParty()
            {
                PartyId = recipientRef
            };

            Email email = new Email()
            {
                From = new ActivityParty[] { fromParty },
                To = new ActivityParty[] { toParty },
                Subject = "Email test",
                Description = "<div>This is an email test!</div>",
                RegardingObjectId = regardingObjectRef,
                DirectionCode = true //Sets the direction to "Outgoing"
            };

            Guid emailId = service.Create(email);

            //Add a custom attachment from the server

            FileInfo file = new FileInfo(@"C:\Temp\Logo.png");

            if (file.Exists)
            {
                byte[] bytes = File.ReadAllBytes(file.FullName);
                string fileContent = Convert.ToBase64String(bytes);

                ActivityMimeAttachment attachment = new ActivityMimeAttachment()
                {
                    ObjectId = new EntityReference("email", emailId),
                    ObjectTypeCode = "email",
                    FileName = file.Name,
                    MimeType = "image/png",
                    Body = fileContent
                };

                service.Create(attachment);
            }

            SendEmailRequest req = new SendEmailRequest()
            {
                EmailId = emailId,
                TrackingToken = "",
                IssueSend = true
            };

            //Send the email
            SendEmailResponse res = (SendEmailResponse)service.Execute(req);
        }
    }
}