Embed images when sending HTML E-Mail

[ChargingBulle]

external resources are not automatically loaded in sanely configured E-Mail clients.

For example in Thunderbird a warning like this will pop up:

the NextCloud logo is fetched from https://yourserverhere.com/core/img/logo.png?v=3D0 which is opens the door for resource based tracking.

It's not just the NextCloud logo but all image resources. For example in an mail based invite the small icons like and will not be displayed when viewed in a many E-Mail client (for example Thunderbird with default settings)

Why change this?

• privacy (readers can be sure that the NC admin does not track them)
• usability (readers can see the images without clicking away a warning prompt)

Is this technically possible?

• Yes, you can embed images. For example via MIME Content-ID or via inline embedding

Technical Example

Black and white image reading "Hello" is inserted into an HTML E-Mail via Content-ID. The image is base64 encoded inside the E-Mail  ``` Return-path: Received: ... Original-recipient: rfc822;user@example.com Sender: user@example.com To: Me From: Me Subject: Test Message-id: <123e4567-e89b-12d3-a456-426655440000@example.com> Date: Sun, 13 Jan 2019 00:00:00 +0000 MIME-version: 1.0 Content-type: multipart/alternative; boundary=------------63E36A7002E8D6FBEB9C5425 Content-language: en-US This is a multi-part message in MIME format. --------------63E36A7002E8D6FBEB9C5425 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit --------------63E36A7002E8D6FBEB9C5425 Content-Type: multipart/related; boundary="------------885E82E985699C9CB72624B8" --------------885E82E985699C9CB72624B8 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 7bit --------------885E82E985699C9CB72624B8 Content-Type: image/png; name="poclpdkelpocpgpa.png" Content-Transfer-Encoding: base64 Content-ID: Content-Disposition: inline; filename="poclpdkelpocpgpa.png" iVBORw0KGgoAAAANSUhEUgAAADgAAAASCAIAAACB04oZAAAAzUlEQVRIie2T0REDIQhEqYuC qIdqaGaLMR9Rz1PGU+NMLpPbP0HXByKFHxF9G2BU/whqQlGsmMxeqwTNXqwIAcpxKTbjCOUO Sj87CBptysCC7S1ATVKXiZjFvBsXQaGHN0u7ZQIUyiyGvDRldqZiBdTkHIVJbd2AUqVsYNIO qxdbAHViTcuGO+rUEAv5GNQrtwmOPz08u0GUi+zWjoY0SYcjTHnL09fOQDv+VO4+zWXx1vkM TMt/L3qY+aORTvaz7x0zv/7GekB36wHdrRcZX4h7tVmUbQAAAABJRU5ErkJggg== --------------885E82E985699C9CB72624B8-- --------------63E36A7002E8D6FBEB9C5425-- ```

[pirate]

The images appear entirely broken for us in Gmail as well.

[jancborchardt]

@JavaZauber @pirate @mokken would you like to contribute a fix for this? Or do you have any pointers what is necessary to fix this? :)

[narcisgarcia]

Easier alternative for developer: Not using images at all. Composition seems to be implemented with IEMailTemplate.php

[joshtrichards]

Unfortunately when I looked at this awhile back I found inline embeds to be an obstacle if one care's about recipients using, say, Gmail. I don't recall finding a good path forward so that effort stalled out.

Similar to: https://github.com/nextcloud/server/issues/17187#issuecomment-692551296