eaf-open-mail-as-html doesn't display embedded images that are attachments

**Describe the bug**
When the html version of an email incorporates pictures that are attachments in the email, the eaf-open-mail-as-html view only displays broken image areas for those pictures.

**To Reproduce**
1. Open email with inline images that are attachments.
2. Call eaf-open-mail-as-html

**Expected behavior**
It will display all images correctly in the html view.

**Versions (please complete the following info):**
- Distro and DE/WM: Turnkey Core 16.0 and Xpra
- Versions of Dependencies:
- - python-pyqt5: 5.11.3+dfsg-1+b3
- - python-dbus: 1.2.8-3
- - python-pyqtwebengine: 5.11.3+dfsg-1+b3
- M-x emacs-version: GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars) of 2019-09-23, modified by Debian

**Screenshots**

**Additional context**
Add any other context about the problem here.