There is only one file type that uses the .P7S extension:
Digitally Signed Email Message
A P7S file is an email message that includes a digital signature. It can be used for sending secure emails that can only be viewed by the intended recipient. P7S files verify that the email is from who it claims to be from and that the email has not been modified in transit.
Most email applications, such as Microsoft Outlook, Apple Mail, and Mozilla Thunderbird, support digital signatures. However, if an email program does not handle digital signatures, a P7S file may appear as an attachment (smime.p7s) to the email message.
NOTE: Email programs that use P7S files use PKCS (Public-Key Cryptography Standard) #7 to sign email messages.
Updated: October 18, 2016