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fix(smtp): embed inline attachments as linked resources #47

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fix(smtp): embed inline attachments as linked resources #47

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37 changes: 37 additions & 0 deletions src/Senders/FluentEmail.Smtp/SmtpSender.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
using FluentEmail.Core.Interfaces;
using FluentEmail.Core.Models;
using System;
using System.Linq;
using System.Net.Mail;
using System.Text;
using System.Threading;
Expand Down Expand Up @@ -77,6 +78,7 @@ private MailMessage CreateMailMessage(IFluentEmail email)
{
var data = email.Data;
MailMessage message = null;
AlternateView htmlView = null;

// Smtp seems to require the HTML version as the alternative.
if (!string.IsNullOrEmpty(data.PlaintextAlternativeBody))
Expand All @@ -92,6 +94,22 @@ private MailMessage CreateMailMessage(IFluentEmail email)
var mimeType = new System.Net.Mime.ContentType("text/html; charset=UTF-8");
AlternateView alternate = AlternateView.CreateAlternateViewFromString(data.Body, mimeType);
message.AlternateViews.Add(alternate);
htmlView = alternate;
}
else if (data.IsHtml && data.Attachments.Any(IsLinkedResource))
{
// Inline images can only be embedded through an AlternateView's
// LinkedResources, so the HTML body goes into one.
message = new MailMessage
{
Subject = data.Subject,
SubjectEncoding = Encoding.UTF8,
From = new MailAddress(data.FromAddress.EmailAddress, data.FromAddress.Name)
};

var mimeType = new System.Net.Mime.ContentType("text/html; charset=UTF-8");
htmlView = AlternateView.CreateAlternateViewFromString(data.Body, mimeType);
message.AlternateViews.Add(htmlView);
}
else
{
Expand Down Expand Up @@ -146,6 +164,22 @@ private MailMessage CreateMailMessage(IFluentEmail email)

data.Attachments.ForEach(x =>
{
// An inline attachment referenced from the HTML (<img src="cid:...">)
// becomes a LinkedResource of the HTML view, which produces
// multipart/related. Added as a regular attachment it would be sent
// as multipart/mixed with "Content-Disposition: attachment", and
// mail clients show it as a separate attachment.
if (htmlView != null && IsLinkedResource(x))
{
var resource = x.ContentType == null
? new LinkedResource(x.Data)
: new LinkedResource(x.Data, x.ContentType);
resource.ContentId = x.ContentId;
resource.ContentType.Name = x.Filename;
htmlView.LinkedResources.Add(resource);
return;
}

System.Net.Mail.Attachment a = new System.Net.Mail.Attachment(x.Data, x.Filename, x.ContentType);

a.ContentId = x.ContentId;
Expand All @@ -155,6 +189,9 @@ private MailMessage CreateMailMessage(IFluentEmail email)

return message;
}

private static bool IsLinkedResource(FluentEmail.Core.Models.Attachment attachment) =>
attachment.IsInline && !string.IsNullOrEmpty(attachment.ContentId);
}

// Taken from https://stackoverflow.com/questions/28333396/smtpclient-sendmailasync-causes-deadlock-when-throwing-a-specific-exception/28445791#28445791
Expand Down
82 changes: 82 additions & 0 deletions test/FluentEmail.Core.Tests/SmtpSenderTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,6 +126,88 @@ public void CanSendHtmlAndPlaintextTogether()
(response.Successful).Should().BeTrue();
}

private static Attachment InlineImage() => new()
{
Data = new MemoryStream([0x89, 0x50, 0x4E, 0x47]),
ContentType = "image/png",
Filename = "logo.png",
ContentId = "logo",
IsInline = true
};

private static string ReadSentEmail(string tempDirectory) =>
File.ReadAllText(Directory.EnumerateFiles(tempDirectory, "*.eml").Single());

[Fact]
public async Task InlineAttachmentIsEmbeddedInHtmlBody()
{
var email = TestEmail
.Body("<p><img src=\"cid:logo\"></p>", true)
.Attach(InlineImage());

email.Sender = GetSender(out var s);

var response = await email.SendAsync();
var eml = ReadSentEmail(s);

DeleteTemp(s);

(response.Successful).Should().BeTrue();
eml.Should().Contain("Content-Type: multipart/related");
eml.Should().Contain("Content-ID: <logo>");
eml.Should().NotContain("Content-Disposition: attachment");
}

[Fact]
public async Task InlineAttachmentIsEmbeddedInHtmlAlternateView()
{
var email = TestEmail
.Body("<p><img src=\"cid:logo\"></p>", true)
.PlaintextAlternativeBody("Plain text version")
.Attach(InlineImage());

email.Sender = GetSender(out var s);

var response = await email.SendAsync();
var eml = ReadSentEmail(s);

DeleteTemp(s);

(response.Successful).Should().BeTrue();
eml.Should().Contain("Content-Type: multipart/alternative");
eml.Should().Contain("Content-Type: multipart/related");
eml.Should().Contain("Content-ID: <logo>");
eml.Should().NotContain("Content-Disposition: attachment");
}

[Fact]
public async Task RegularAttachmentStaysAttachmentNextToInlineAttachment()
{
var email = TestEmail
.Body("<p><img src=\"cid:logo\"></p>", true)
.Attach(InlineImage())
.Attach(new Attachment
{
Data = new MemoryStream("some text"u8.ToArray()),
ContentType = "text/plain",
Filename = "report.txt"
});

email.Sender = GetSender(out var s);

var response = await email.SendAsync();
var eml = ReadSentEmail(s);

DeleteTemp(s);

(response.Successful).Should().BeTrue();
eml.Should().Contain("Content-Type: multipart/mixed");
eml.Should().Contain("Content-Type: multipart/related");
eml.Should().Contain("Content-ID: <logo>");
eml.Should().Contain("Content-Type: text/plain; name=report.txt");
eml.Split("Content-Disposition: attachment").Length.Should().Be(2, "only report.txt is a regular attachment");
}

[Fact]
public void CancelSendIfCancellationRequested()
{
Expand Down