add APNG support (part 1)

references:
https://wiki.mozilla.org/APNG_Specification#Structure
http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html

Author: CatKasha

filetype/types/image.py

@@ -48,6 +48,46 @@ def match(self, buf):
         )
 
 
+class Apng(Type):
+    """
+    Implements the APNG image type matcher.
+    """
+    MIME = 'image/apng'
+    EXTENSION = 'apng'
+
+    def __init__(self):
+        super(Apng, self).__init__(
+            mime=Apng.MIME,
+            extension=Apng.EXTENSION
+        )
+
+    def match(self, buf):
+        if(len(buf) > 8 and
+           buf[:8] == bytearray([0x89, 0x50, 0x4e, 0x47,
+                                 0x0d, 0x0a, 0x1a, 0x0a])):
+            #cursor in buf, skip already readed 8 bytes
+            i = 8
+            while len(buf) > i:
+                data_length = int.from_bytes(buf[i:i+4], byteorder="big")
+                i += 4
+
+                chunk_type = buf[i:i+4].decode("ascii")
+                i += 4
+
+                #acTL chunk in APNG should appears first than IDAT
+                #IEND is end of PNG
+                if (chunk_type == "IDAT" or chunk_type == "IEND"):
+                    return 0
+                elif (chunk_type == "acTL"):
+                    return 1
+
+                #move to the next chank by skipping data and crc (4 bytes)
+                i += data_length + 4
+            return 0
+        else:
+            return 0
+
+
 class Png(Type):
     """
     Implements the PNG image type matcher.