Improve DexFile FILE_TYPE_RE and add FILE_TYPE_HEADER_PREFIX.

diffoscope/comparators/dex.py

@@ -60,7 +60,8 @@ class DexContainer(Archive):
 
 class DexFile(File):
     DESCRIPTION = "Dalvik .dex files"
-    FILE_TYPE_RE = re.compile(r"^Dalvik dex file .*\b")
+    FILE_TYPE_HEADER_PREFIX = b"dex\n"
+    FILE_TYPE_RE = re.compile(r"^Dalvik dex file\b")
     FILE_EXTENSION_SUFFIX = {".dex"}
     CONTAINER_CLASSES = [DexContainer]