Attaching Image in the body of mail in C#

    string attachmentPath = Environment.CurrentDirectory + @"\test.png";
    Attachment inline = new Attachment(attachmentPath);
    inline.ContentDisposition.Inline = true;
    inline.ContentDisposition.DispositionType = DispositionTypeNames.Inline;
    inline.ContentId = contentID;
    inline.ContentType.MediaType = "image/png";
    inline.ContentType.Name = Path.GetFileName(attachmentPath);

    message.Attachments.Add(inline);

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reference: Send an Email in C# with Inline attachments