That's a gzipped (compressed) pcap file, or possibly a gzipped pcapng file. Wireshark/TShark can read those files, but programs such as sngrep, which use libpcap/WinPcap, can't do so unless they do the uncompression themselves, and sngrep doesn't.
Rename it to 12345.pcap.gz, and then do gunzip 12345.pcap.gz; that'll uncompress it into a file named 12345.pcap, which will probably be a pcap or pcapng file, either of which OS X 10.11.4's libpcap can read.