images: use jpg extensions for jpeg files everywhere

fix #2254

beets/art.py

@@ -193,7 +193,7 @@ def extract(log, outpath, item):
         return
 
     # Add an extension to the filename.
-    ext = imghdr.what(None, h=art)
+    ext = mediafile.image_extension(art)
     if not ext:
         log.warning(u'Unknown image type in {0}.',
                     displayable_path(item.path))

beets/mediafile.py

@@ -59,6 +59,7 @@
 
 from beets import logging
 from beets.util import displayable_path, syspath, as_string
+from beets.util.collections import IdentityUnlessDict
 import six
 
 
@@ -81,6 +82,8 @@
     'aiff': 'AIFF',
 }
 
+PREFERRED_IMAGE_EXTENSIONS = IdentityUnlessDict({'jpeg': 'jpg'})
+
 
 # Exceptions.
 
@@ -351,6 +354,10 @@ def image_mime_type(data):
         return 'image/x-{0}'.format(kind)
 
 
+def image_extension(data):
+    return PREFERRED_IMAGE_EXTENSIONS[_imghdr_what_wrapper(data)]
+
+
 class ImageType(enum.Enum):
     """Indicates the kind of an `Image` stored in a file's tag.
     """

beets/util/collections.py

@@ -0,0 +1,6 @@
+class IdentityUnlessDict(dict):
+    def __getitem__(self, key):
+        try:
+            return dict.__getitem__(self, key)
+        except KeyError:
+            return key