Hi Mlancelin,
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", highlight Attach File and click Add button. Click OK.
Then, you can see this button:
Then, you can see "Insert as Text" option come back.
Thanks,
Richard