T.38 Fax over IP (FoIP)

# T.38 is a network protocol, not a standard file. To detect it in a network capture, you would parse the SDP using a library like pyshark:
import pyshark
cap = pyshark.FileCapture('fax_call.pcap', display_filter='sdp')
for packet in cap:
    if 'image' in packet.sdp.media_type and 't38' in packet.sdp.media_format:
        print('T.38 Fax Negotiation Detected!')

// Detecting T.38 negotiation in a raw SIP SDP string:
const sdp = 'v=0\r\nm=image 10000 udptl t38\r\na=T38FaxVersion:0\r\n';
const isT38 = /^m=image \d+ udptl t38/m.test(sdp);
console.log(isT38); // true

import "strings"
// Checking for T.38 media descriptor in an SDP payload
func isT38FoIP(sdp string) bool {
    return strings.Contains(sdp, "m=image") && strings.Contains(sdp, "t38")
}

// Standard file detection like mime_content_type() will not work on raw T.38 network dumps. It requires SIP/SDP parser logic.