Attachments sent by your leads cannot be viewed directly in the Unibox/Master Inbox of your email sending tool.
To view the attachments, you'll need to access your email mailbox.
How?
You can use any email client software or open-source software such as Thunderbird or Apple Mail.
You can refer to the two tutorials below👇:
Apple mail: How to log in to your MailScale inboxes
Thunderbird: