What Outlook version are you using?
If you are using Outlook 2013 or previous versions, please follow these steps:
- open a new email
- locate Insert > Attach File
- choose the html file and select Insert as Text
If you are using Outlook 2016 or later versions, the Insert as Text option is hidden and invisible.
Here is the workaround:
- open a new email
- locate File > Options >Quick Access Toolbar
- add Attach File to Quick Access Toolbar
- open Attach File via Quick Access Toolbar and the Insert as Text option will reappear
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