I'm surprised you haven't hit this before now. This is a Thunderbird setting (and it exists in other e-mail clients) where if the word attach or attachment is encountered when typing text, and you do not have an attachment on the message, it flags that to prevent you from forgetting to put it on to the message.
This feature can be turned off in Thunderbird. If you open Options and go to the Composition tab there is a checkbox for Check for missing attachments that should be unchecked.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore