There is only one file type that uses the .P10 extension:
Certificate Request File
A P10 file is used by programs that include public-key cryptography authentication mechanisms. It contains a request, which asks the receiving entity, usually a certificate authority (CA), to digitally sign a file. P10 files are created using the PKCS#10 (Public-Key Cryptography Standards #10) standard.
P10 files are similar to .CSR files.
Updated: April 8, 2016