我尝试使用HTML文件发送电子邮件进行结构。但它返回了纯文本

问：

所以我对编程很陌生，负责发送电子邮件。电子邮件被发送。 但似乎我要么不使用 HTML。或者我以某种方式重写或删除它。有人可以帮我了解我做错了什么吗？我附上了发送电子邮件的一段代码。如果您需要更多，请告诉我

if (oInf.Error == 0)
{
    string templateFilePath = AppDomain.CurrentDomain.BaseDirectory + "spwebfiles\\template.html";
    string template = "";

    using (StreamReader rd = new StreamReader(templateFilePath))
    {
        template = rd.ReadToEnd();
    }
//all these values comes from an an LIstobj, asume they are already filled.
    StringBuilder emailBody = new StringBuilder();
    emailBody.AppendLine($"Cliente: {companyName}<br>");
    emailBody.AppendLine("Módulo: Inventario<br>");
    emailBody.AppendLine($"Estación: {stationId}<br>");
    emailBody.AppendLine("<br>");

    foreach (var InventoryOffline_Model in oInf.ListObj)
    {
///sending a block of code, this will happen n# times as i might send several items or just 1
        string docName = THelperApp.GetDocName(InventoryOffline_Model.TranType);
        string block = string.Format(
            "<br>Fecha de creación del documento: {0} <br>Tipo de Documento: {1} <br>Mensaje de Error: {2} <br>" +
            "ID del documento: {3} <br>Intentos de Sincronización: {4} <br>",
            InventoryOffline_Model.DateE,
            docName,
            InventoryOffline_Model.MessageSync,
            InventoryOffline_Model.OfflineDocumentId,
            InventoryOffline_Model.Retry);

        emailBody.AppendLine(block); 
///sending a block of code, this will happen n# times as i might send several items or just 1
    }

    THelperSendMail.SendGrid("MyCompanyEmail.com",
        SendTo, //got this from appsetings
        "Alertas de sincronización",
        emailBody.ToString(),//taking the block as text. maybe this is the mistake?
        "My Company",
        true);

    if (THelperSendMail.Error)
    {
        _oResp.SendError(-248, "Ha ocurrido un error en el envío de correo electrónico");
    }
    else
    {
        _oResp.SendResponse(message: "");
    }

使用 HTML 文件作为正文发送电子邮件

电子邮件以纯文本形式发送

HTML 文件永远不会被使用或在发送电子邮件时被重写