diff --git a/src/calibre/__init__.py b/src/calibre/__init__.py
index 1799072045..2f457bf2bc 100644
--- a/src/calibre/__init__.py
+++ b/src/calibre/__init__.py
@@ -217,14 +217,25 @@ def filename_to_utf8(name):
     return name.decode(codec, 'replace').encode('utf8')
 
 def extract(path, dir):
-    ext = os.path.splitext(path)[1][1:].lower()
     extractor = None
-    if ext in ['zip', 'cbz', 'epub', 'oebzip']:
-        from calibre.libunzip import extract as zipextract
-        extractor = zipextract
-    elif ext in ['cbr', 'rar']:
+    # First use the file header to identify its type
+    with open(path, 'rb') as f:
+        id_ = f.read(3)
+    if id_ == b'Rar':
         from calibre.libunrar import extract as rarextract
         extractor = rarextract
+    elif id_.startswith(b'PK'):
+        from calibre.libunzip import extract as zipextract
+        extractor = zipextract
+    if extractor is None:
+        # Fallback to file extension
+        ext = os.path.splitext(path)[1][1:].lower()
+        if ext in ['zip', 'cbz', 'epub', 'oebzip']:
+            from calibre.libunzip import extract as zipextract
+            extractor = zipextract
+        elif ext in ['cbr', 'rar']:
+            from calibre.libunrar import extract as rarextract
+            extractor = rarextract
     if extractor is None:
         raise Exception('Unknown archive type')
     extractor(path, dir)
diff --git a/src/calibre/customize/builtins.py b/src/calibre/customize/builtins.py
index 1e40a8e5ff..91abfacc95 100644
--- a/src/calibre/customize/builtins.py
+++ b/src/calibre/customize/builtins.py
@@ -166,6 +166,14 @@ class ComicMetadataReader(MetadataReaderPlugin):
     description = _('Extract cover from comic files')
 
     def get_metadata(self, stream, ftype):
+        if hasattr(stream, 'seek') and hasattr(stream, 'tell'):
+            pos = stream.tell()
+            id_ = stream.read(3)
+            stream.seek(pos)
+            if id_ == b'Rar':
+                ftype = 'cbr'
+            elif id.startswith(b'PK'):
+                ftype = 'cbz'
         if ftype == 'cbr':
             from calibre.libunrar import extract_first_alphabetically as extract_first
             extract_first