The automatically downloaded attachments and inline images, unless you've "saved" them (under the three dots) will be automatically cleaned up by the app over time.
And just to check, those are stored in /Android/data/org.kman.AquaMail/files/parts.
Anything under /Downloads is attachments you've clicked and "save"'d explicitly, and those (unlike the above) are under your control and won't be touched by the app.
