In the email currently being viewed, click on "Show more" in the top right corner, and then select "Attachment management" from the dropdown menu.
In the pop-up window, a list of all attachments in the email is displayed. Check the files you want to delete, then click "Delete" to complete the action of deleting the attachments.
Remarks
To delete a single file, you can also directly click on the "Delete" button on the right side of the file.