Mass Mail tool attachment strips the file

Hi
Using Joomla 3.10.x and EB 3.17.16
Client is using the Mass mail tool to send notifications and includes a pdf attachment. 
The message comes through to my inbox but the pdf is renamed and stripped of the .pdf suffix making it difficult for users to open.
They are using Elastic Email  SMTP for mail.
Any ideas if its EB or Elastic Email causing this behaviour?
Thanks

I looked at this again and could not see how it could happen. Could you please try to use it on a site without Elastic Email SMTP to see if the issue happens?

Regards,

Tuan