Finally found it !!!
I reply to myself as the title may help others (faster…)
Thanks to @kishanprmr
1/
In the attachments field for the “New Email” trigger, it contains data and fileName properties, as shown in the image.
2/
When you pass these attachments to the “Send Email” action, make sure you fill the “Attachment” and “Attachment Name” fields, as shown in the image.
3/
This will add the attachment with the correct file name.