That behavior matches Outlook’s intelligent suggestion behavior. In Outlook on the web and Outlook.com, Outlook can detect content in a message and show suggestions such as Attach a file. That suggestion can appear when Outlook thinks a file may be relevant to the conversation, and selecting it creates a draft with the chosen attachment.
If the suggestion is not helpful, it can be turned off:
New Outlook on the web
- Select Settings > Mail > Smart suggestions.
- Clear Show suggested replies.
- Select Save.
Classic Outlook on the web
- Select Settings > Mail > Suggested replies.
- Clear Show suggested replies.
- Select Save.
If the goal is to report this as product feedback, the documented feedback channel is Provide feedback.