The customer gets from his system random hexadecimal filenmames which end with .p7m when exporing their mails.
Since usually when a file is encrypted with CMS it gets the .p7m extension added normal files should not be affected. But at least we should have some kind of error detection if what we receive after decryption does not look like a mail at all but some random binary data.
Makes sense to me because with normal kleo decrypting an foo.p7m would leave you with just a file named "foo" and then windows will not know what to do with it since there is no MIME Database / File style inspection of the contents happening