Improve aws-s3 gzip file detection to avoid false negatives (#29969)

Directly check the byte stream for the gzip magic number and deflate
compression type. Avoid using http.DetectContentType because it returns
the first match it finds while checking many signatures.

Closes #29968

(cherry picked from commit 61a7d36)

CHANGELOG.next.asciidoc

@@ -53,6 +53,12 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 *Filebeat*
 
 - aws-s3: Stop trying to increase SQS message visibility after ReceiptHandleIsInvalid errors. {pull}29480[29480]
+- Fix handling of IPv6 addresses in netflow flow events. {issue}19210[19210] {pull}29383[29383]
+- Fix `sophos` KV splitting and syslog header handling {issue}24237[24237] {pull}29331[29331]
+- Undo deletion of endpoint config from cloudtrail fileset in {pull}29415[29415]. {pull}29450[29450]
+- Make Cisco ASA and FTD modules conform to the ECS definition for event.outcome and event.type. {issue}29581[29581] {pull}29698[29698]
+- ibmmq: Fixed `@timestamp` not being populated with correct values. {pull}29773[29773]
+- aws-s3: Improve gzip detection to avoid false negatives. {issue}29968[29968]
 
 *Heartbeat*
 

x-pack/filebeat/input/awss3/s3_objects.go

@@ -15,7 +15,6 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
-	"net/http"
 	"reflect"
 	"strings"
 	"time"
@@ -375,18 +374,13 @@ func s3ObjectHash(obj s3EventV2) string {
 // stream without consuming it. This makes it convenient for code executed after this function call
 // to consume the stream if it wants.
 func isStreamGzipped(r *bufio.Reader) (bool, error) {
-	// Why 512? See https://godoc.org/net/http#DetectContentType
-	buf, err := r.Peek(512)
+	buf, err := r.Peek(3)
 	if err != nil && err != io.EOF {
 		return false, err
 	}
 
-	switch http.DetectContentType(buf) {
-	case "application/x-gzip", "application/zip":
-		return true, nil
-	default:
-		return false, nil
-	}
+	// gzip magic number (1f 8b) and the compression method (08 for DEFLATE).
+	return bytes.HasPrefix(buf, []byte{0x1F, 0x8B, 0x08}), nil
 }
 
 // s3Metadata returns a map containing the selected S3 object metadata keys.