diff --git a/beets/library.py b/beets/library.py
index 33087de87..789b4c72f 100644
--- a/beets/library.py
+++ b/beets/library.py
@@ -397,7 +397,7 @@ class Item(LibModel):
         except (OSError, IOError) as exc:
             raise ReadError(read_path, exc)
 
-        for key in ITEM_KEYS_META:
+        for key in list(ITEM_KEYS_META) + MediaFile.custom_fields:
             value = getattr(f, key)
             if isinstance(value, (int, long)):
                 # Filter values wider than 64 bits (in signed
diff --git a/beets/mediafile.py b/beets/mediafile.py
index 3f207ef20..7c47e54f2 100644
--- a/beets/mediafile.py
+++ b/beets/mediafile.py
@@ -1276,6 +1276,8 @@ class MediaFile(object):
                          'channels', 'format']:
             yield property
 
+    custom_fields = []
+
     @classmethod
     def add_field(cls, name, descriptor):
         """Add a field to store custom tags.
@@ -1293,6 +1295,7 @@ class MediaFile(object):
         if name in cls.__dict__:
             raise ValueError(
                 u'property "{0}" already exists on MediaField'.format(name))
+        cls.custom_fields.append(name)
         setattr(cls, name, descriptor)
 
     def update(self, dict, id3v23=False):
diff --git a/test/test_mediafile.py b/test/test_mediafile.py
index 5aa669bd5..4334f3c7e 100644
--- a/test/test_mediafile.py
+++ b/test/test_mediafile.py
@@ -272,6 +272,7 @@ class ExtendedFieldTestMixin(object):
         mediafile = MediaFile(mediafile.path)
         self.assertEqual(mediafile.initialkey, 'F#')
         delattr(MediaFile, 'initialkey')
+        MediaFile.custom_fields = []
 
     def test_extended_attribute_from_item(self):
         MediaFile.add_field('initialkey', field_extension)
@@ -284,6 +285,19 @@ class ExtendedFieldTestMixin(object):
         mediafile = MediaFile(mediafile.path)
         self.assertEqual(mediafile.initialkey, 'Gb')
         delattr(MediaFile, 'initialkey')
+        MediaFile.custom_fields = []
+
+    def test_flexible_attribute_from_file(self):
+        MediaFile.add_field('initialkey', field_extension)
+
+        mediafile = self._mediafile_fixture('empty')
+        mediafile.update({'initialkey': 'F#'})
+
+        item = Item.from_path(mediafile.path)
+        self.assertEqual(item['initialkey'], 'F#')
+
+        delattr(MediaFile, 'initialkey')
+        MediaFile.custom_fields = []
 
     def test_invalid_descriptor(self):
         with self.assertRaises(ValueError) as cm: