Hi @Bryan Arciniega ,
As far as I know, the Attach File menu button in Outlook 2016 is a new feature to easily allow you to insert files that are stored in the Cloud like OneDrive and OneDrive for Business or a corporate SharePoint server.
And the “Insert as Text” was canceled for this new “Attach File” button, as a workaround, please add the classic “Attach File” button:
Click New Email button to open a new email message, click File > Options > Quick Access Toolbar. Under "Popular Commands", select Attach File and click Add button. Click OK.
You will see the "attach a file" in the quick access toolbar, click the button to open a new window.
Select the htm file, press the down arrow on the Insert button > Insert as Text
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.