Group attachment in output node mail-parse

Hi all,
I'm using the node node-red-contrib-mail-parse to parse the eml file.
This node has two output, the first output returns in msg.payload an object with parsed email.
The second output returns in msg.payload the single attachments in parallel.
I want to group the attachments of the second output in an array.
I'm using the join node to make this array and it work fine if I pass the number of the total attachments, but in real use case I can't insert the number of attachments so I'm trying to use the msg.complete but I can't to reach my scope because te output of the node-red-contrib-mail-parse are single payload in parallel and I don't know to count the attachments to set the msg.complete at the last.
Do you have an idea?