I just sent an email that included the text "I attached a photo of it" to a client, but i ended up forgetting to attach the photo attachment. In gmail it will remind you if you have the keyword "attachment" in your email when pressing send. I'd like that gmail feature to be in Proton too. This request is just a simple regex and warning message on pressing send for either the keyword "attachment" or a key phrase in this string of text: "I attached a photo of it" when there is no attachment attached to the email.

The email i sent was an HTML email but still that shouldn't weaken this detection.