mu4e lets you define custom actions for messages in The headers view
and for both messages and attachments in The message view. Custom
actions allow you to easily extend mu4e for specific needs — for example,
marking messages as spam in a spam filter or applying an attachment with a
source code patch.
You can invoke the actions with key a for actions on messages, and key A for actions on attachments.
For general information extending mu4e and writing your own functions, see
Extending mu4e.