Create a new email in Outlook by clicking the "New E-mail" button. Click the "Format Text" tab. Most tools in the toolbox are grayed out; to make them available, click inside the email body box.

Click the "Insert" button and you should then see the file in a new "Attached:" line in your email. (If you change your mind and want to remove the attachment, right-click on it and select "Remove ...

Hints related to the feature were spotted in v2024.03.31.621006929 of the Gmail app. Check out the screenshot below, showing the relevant (but inactive) button below the email title. AssembleDebug ...