Distinguish between M4{A,B,P} formats.

Note: Removes tag.AAC.

id.go

@@ -26,8 +26,20 @@ func Identify(r io.ReadSeeker) (format Format, fileType FileType, err error) {
 	case string(b[0:4]) == "OggS":
 		return VORBIS, OGG, nil
 
-	case string(b[4:11]) == "ftypM4A":
-		return AAC, MP4, nil
+	case string(b[4:8]) == "ftyp":
+		b = b[8:11]
+		fileType = UnknownFileType
+		switch string(b) {
+		case "M4A":
+			fileType = M4A
+
+		case "M4B":
+			fileType = M4B
+
+		case "M4P":
+			fileType = M4P
+		}
+		return MP4, fileType, nil
 
 	case string(b[0:3]) == "ID3":
 		b := b[3:]