diff --git a/README.md b/README.md
index 7c8161d..1fa1ba9 100644
--- a/README.md
+++ b/README.md
@@ -8,8 +8,11 @@
 A MIME type sniffer for Go.
 
 MIMESniffer implements the algorithm described at
-[here](https://mimesniff.spec.whatwg.org) to determine the MIME type of the
-given data. So it can be used as a substitute for `http.DetectContentType()`.
+[here](https://mimesniff.spec.whatwg.org) and uses the file signatures (aka
+[magic numbers](https://en.wikipedia.org/wiki/Magic_number_\(programming\)#Magic_numbers_in_files))
+listed [here](https://www.garykessler.net/library/file_sigs.html) to determine
+the MIME type of the given data. So it can be used as an alternative for the
+`http.DetectContentType()`.
 
 ## Features
 
diff --git a/mimesniffer.go b/mimesniffer.go
index 6866d9e..b7bcc76 100644
--- a/mimesniffer.go
+++ b/mimesniffer.go
@@ -78,10 +78,8 @@ func Register(mimeType string, sniffer func([]byte) bool) {
 // It always returns a valid MIME type: if it cannot determine a more specific
 // one, it returns "application/octet-stream".
 func Sniff(b []byte) string {
-	const unknownType = "application/octet-stream"
-
 	if len(b) == 0 {
-		return unknownType
+		return "application/octet-stream"
 	}
 
 	for mt, s := range registeredSniffers {
@@ -90,17 +88,13 @@ func Sniff(b []byte) string {
 		}
 	}
 
-	if mt := http.DetectContentType(b); mt != unknownType {
-		return mt
-	}
-
 	for mt, s := range defaultSniffers {
 		if s(b) {
 			return mt
 		}
 	}
 
-	return unknownType
+	return http.DetectContentType(b)
 }
 
 // applicationEPUBZip reports whether the b's MIME type is
diff --git a/mimesniffer_test.go b/mimesniffer_test.go
index 8450c75..8b260dd 100644
--- a/mimesniffer_test.go
+++ b/mimesniffer_test.go
@@ -32,6 +32,7 @@ func TestSniff(t *testing.T) {
 	})
 
 	assert.Equal(t, Sniff([]byte{0x00}), "foo/bar")
-	assert.Equal(t, Sniff([]byte("foobar")), "text/plain; charset=utf-8")
 	assert.Equal(t, Sniff([]byte{0x01}), "application/octet-stream")
+	assert.Equal(t, Sniff([]byte{0xff, 0xf1}), "audio/aac")
+	assert.Equal(t, Sniff([]byte("foobar")), "text/plain; charset=utf-8")
 }