MediaFile: improve cover art detection

Filter media file images on their type.
Detection is still not deterministic when 0 or multiple image
have type ImageType.front.

Fix #1332.

beets/mediafile.py

@@ -1246,18 +1246,31 @@ class DateItemField(MediaField):
 
 class CoverArtField(MediaField):
     """A descriptor that provides access to the *raw image data* for the
-    first image on a file. This is used for backwards compatibility: the
+    cover image on a file. This is used for backwards compatibility: the
     full `ImageListField` provides richer `Image` objects.
+
+    When there are multiple images we try to pick the most likely to be a front
+    cover.
     """
     def __init__(self):
         pass
 
     def __get__(self, mediafile, _):
-        try:
-            return mediafile.images[0].data
-        except IndexError:
+        candidates = mediafile.images
+        if candidates:
+            return self.guess_cover_image(candidates).data
+        else:
             return None
 
+    @staticmethod
+    def guess_cover_image(candidates):
+        if len(candidates) == 1:
+            return candidates[0]
+        try:
+            return next(c for c in candidates if c.type == ImageType.front)
+        except StopIteration:
+            return candidates[0]
+
     def __set__(self, mediafile, data):
         if data:
             mediafile.images = [Image(data=data)]

test/test_mediafile.py

@@ -28,7 +28,7 @@ from test import _common
 from test._common import unittest
 from beets.mediafile import MediaFile, MediaField, Image, \
     MP3DescStorageStyle, StorageStyle, MP4StorageStyle, \
-    ASFStorageStyle, ImageType
+    ASFStorageStyle, ImageType, CoverArtField
 from beets.library import Item
 from beets.plugins import BeetsPlugin
 
@@ -161,6 +161,13 @@ class ImageStructureTestMixin(ArtTestMixin):
         mediafile = MediaFile(mediafile.path)
         self.assertEqual(len(mediafile.images), 0)
 
+    def test_guess_cover(self):
+        mediafile = self._mediafile_fixture('image')
+        self.assertEqual(len(mediafile.images), 2)
+        cover = CoverArtField.guess_cover_image(mediafile.images)
+        self.assertEqual(cover.desc, 'album cover')
+        self.assertEqual(mediafile.art, cover.data)
+
     def assertExtendedImageAttributes(self, image, **kwargs):
         """Ignore extended image attributes in the base tests.
         """
@@ -758,6 +765,10 @@ class MP4Test(ReadWriteTestBase, PartialTestMixin,
         with self.assertRaises(ValueError):
             mediafile.images = [Image(data=self.tiff_data)]
 
+    def test_guess_cover(self):
+        # There is no metadata associated with images, we pick one at random
+        pass
+
 
 class AlacTest(ReadWriteTestBase, unittest.TestCase):
     extension = 'alac.m4a'