Allow me to view "hidden" attachments

There is a special view mode for messages which allows you to see all of a message's attachments, even "hidden" ones, in its attachments list. See Viewing All Body Parts for a detailed explanation of when you might need to do this.

Because this view mode is not needed by most users and could be confusing to some, it is disabled by default. To enable it, install the "Show All Body Parts" add-on.