diff --git a/pkg/ccl/backupccl/backup_test.go b/pkg/ccl/backupccl/backup_test.go
index 9561d4aba35b..c1e9d3ade54c 100644
--- a/pkg/ccl/backupccl/backup_test.go
+++ b/pkg/ccl/backupccl/backup_test.go
@@ -17,7 +17,6 @@ import (
 	"io"
 	"io/ioutil"
 	"math/rand"
-	"net/http"
 	"net/url"
 	"os"
 	"path"
@@ -329,8 +328,7 @@ func TestBackupRestoreStatementResult(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		fileType := http.DetectContentType(backupManifestBytes)
-		require.Equal(t, ZipType, fileType)
+		require.True(t, isGZipped(backupManifestBytes))
 	})
 
 	sqlDB.Exec(t, "CREATE DATABASE data2")
@@ -462,8 +460,7 @@ func TestBackupRestorePartitioned(t *testing.T) {
 					if err != nil {
 						t.Fatal(err)
 					}
-					fileType := http.DetectContentType(backupPartitionBytes)
-					require.Equal(t, ZipType, fileType)
+					require.True(t, isGZipped(backupPartitionBytes))
 				}
 			}
 		}
@@ -1577,8 +1574,7 @@ func TestBackupRestoreResume(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		fileType := http.DetectContentType(backupManifestBytes)
-		if fileType == ZipType {
+		if isGZipped(backupManifestBytes) {
 			backupManifestBytes, err = decompressData(backupManifestBytes)
 			require.NoError(t, err)
 		}
@@ -3935,8 +3931,7 @@ func TestBackupRestoreChecksum(t *testing.T) {
 		if err != nil {
 			t.Fatalf("%+v", err)
 		}
-		fileType := http.DetectContentType(backupManifestBytes)
-		if fileType == ZipType {
+		if isGZipped(backupManifestBytes) {
 			backupManifestBytes, err = decompressData(backupManifestBytes)
 			require.NoError(t, err)
 		}
diff --git a/pkg/ccl/backupccl/manifest_handling.go b/pkg/ccl/backupccl/manifest_handling.go
index 352494c433e2..bf101d9684cc 100644
--- a/pkg/ccl/backupccl/manifest_handling.go
+++ b/pkg/ccl/backupccl/manifest_handling.go
@@ -16,7 +16,6 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io/ioutil"
-	"net/http"
 	"net/url"
 	"path"
 	"sort"
@@ -71,14 +70,26 @@ const (
 const (
 	// BackupFormatDescriptorTrackingVersion added tracking of complete DBs.
 	BackupFormatDescriptorTrackingVersion uint32 = 1
-	// ZipType is the format of a GZipped compressed file.
-	ZipType = "application/x-gzip"
 
 	dateBasedIncFolderName  = "/20060102/150405.00"
 	dateBasedIntoFolderName = "/2006/01/02-150405.00"
 	latestFileName          = "LATEST"
 )
 
+// isGZipped detects whether the given bytes represent GZipped data. This check
+// is used rather than a standard implementation such as http.DetectContentType
+// since some zipped data may be mis-identified by that method. We've seen
+// gzipped data incorrectly identified as "application/vnd.ms-fontobject". The
+// magic bytes are from the MIME sniffing algorithm http.DetectContentType is
+// based which can be found at https://mimesniff.spec.whatwg.org/.
+//
+// This method is only used to detect if protobufs are GZipped, and there are no
+// conflicts between the starting bytes of a protobuf and these magic bytes.
+func isGZipped(dat []byte) bool {
+	gzipPrefix := []byte("\x1F\x8B\x08")
+	return bytes.HasPrefix(dat, gzipPrefix)
+}
+
 // BackupFileDescriptors is an alias on which to implement sort's interface.
 type BackupFileDescriptors []BackupManifest_File
 
@@ -222,8 +233,7 @@ func readBackupManifest(
 		}
 	}
 
-	fileType := http.DetectContentType(descBytes)
-	if fileType == ZipType {
+	if isGZipped(descBytes) {
 		descBytes, err = decompressData(descBytes)
 		if err != nil {
 			return BackupManifest{}, errors.Wrap(
@@ -289,8 +299,7 @@ func readBackupPartitionDescriptor(
 		}
 	}
 
-	fileType := http.DetectContentType(descBytes)
-	if fileType == ZipType {
+	if isGZipped(descBytes) {
 		descBytes, err = decompressData(descBytes)
 		if err != nil {
 			return BackupPartitionDescriptor{}, errors.Wrap(