The recipient receives an encrypted email which has attachments with the name like Attachment1.pgp, Attachment2.pgp. If you know the original format of the attachment file, he still can use the appropriate software to open. For example: Microsoft Word to open the file if it was originally a .doc(x) file.
This is by design by PGP with PGP/Partitioned-, PGP/MIME-encoding.
According to RFC 4880 (section 5.9):
"File name as a string (one-octet length, followed by a file name).This may be a zero-length string. Commonly, if the source of the encrypted data is a file, this will be the name of the encrypted file.
An implementation MAY consider the file name in the Literal packet to be a more authoritative name than the actual file name."
This is in fact an improvement over inline encoding, since it provides more privacy by encrypting the attachment names, which is then stored as a "literal data packet (tag 11)".
If the recipient is using a Symantec-product such as PGP Universal Server, Symantec Encryption Management Server or PGP Desktop, Symantec encryption Desktop then the original file name will be restored correctly when the mail is decrypted.
In case the recipient is using another encryption solution, it may happen that they do not implement the option to restore the original file name by default. In this case, he needs to contact that software provider for a solution.