You can perform custom functions (“actions”) on messages and their attachments. For a general discussion on how to define your own, see Actions.
mu4e-view-action (a) lets you pick some custom action to perform
on the current message. You can specify these actions using the variable
mu4e-view-actions; mu4e defines a number of example actions.
MIME-part actions allow you to act upon MIME-parts in a message - such as
attachments. These actions are defined and documented in
mu4e-view-mime-part-actions.
There are a number of built-in actions which may be a good starting point for creating your own.