Bug: iPhone says "Attachment is over 100MB. You must connect to a WiFi network to send..." - Even though iPhone **IS** connected to WiFi

I need to email a large video (so I can correct the video, which was messed up by different Apple bugs, aarrggg....). Selecting the video, then mail churns for many minutes and creates an email, which iPhone then will not send, popping the above message.

This appears to be a long-standing iOS bug.

Anybody know how to work around it?

Thanks!