The messages which appear in your Trash/Sent Mail are a temporary
artifact of the new-ish Attachment system.
You should find those files disappear within 24 hrs.
If you want to check that out, click on Settings, scroll down to
Attachments, and select the Basic attachment features setting.
Click on Save Changes, and then you're back to selecting one file
at a time. But you shouldn't see those artifacts any more.