Instructions
NOTE: Steps 2-5 only have to be done once in order to set up the "Attach File" button initially. After that, you can simply repeat Steps 1, 6, and 7 in order to insert custom HTML into an email message body.
1) Click the New Email button to begin a new message.
2) Across the top of the new message window, in the Quick Access Toolbar, click the "Customize Quick Access Toolbar" drop-down menu. Choose More Commands from the list of options.
3) In the "Choose commands from" drop-down list, click All Commands.
4) In the list of commands, locate the second one labeled "Attach File", and click it. Then, click the Add button.
NOTE: There are two commands labeled "Attach File". Pick the second one which does not have an ellipsis after it.
5) After you have added the "Attach File" command to the list on the right, click OK to save your changes.
6) At the top of a new email message window, in the Quick Access Toolbar, click the "Attach File" button.
7) Browse to the location of the HTML file that you wish to insert, and click the file name once to select the file. Then, click the drop-down arrow to the right of the "Insert" button and choose Insert as Text.
Outcome
Your HTML file, including all embedded images and formatting, will be inserted into the email message body.