diff --git a/beets/mediafile.py b/beets/mediafile.py
index 48fd0942f..b25184727 100644
--- a/beets/mediafile.py
+++ b/beets/mediafile.py
@@ -251,15 +251,10 @@ def _sc_encode(gain, peak):
 
 
 def _image_mime_type(data):
-    """Return the MIME type (either image/png or image/jpeg) of the
-    image data (a bytestring).
+    """Return the MIME type of the image data (a bytestring).
     """
     kind = imghdr.what(None, h=data)
-    if kind == 'png':
-        return 'image/png'
-    else:
-        # Currently just fall back to JPEG.
-        return 'image/jpeg'
+    return 'image/{0}'.format(kind)
 
 # StorageStyle classes describe strategies for accessing values in
 # Mutagen file objects.
@@ -504,6 +499,8 @@ class MP4ImageStorageStyle(MP4ListStorageStyle):
             kind = mutagen.mp4.MP4Cover.FORMAT_PNG
         elif image.mime_type == 'image/jpeg':
             kind = mutagen.mp4.MP4Cover.FORMAT_JPEG
+        else:
+            raise ValueError('The MP4 format only supports PNG and JPEG images')
         return mutagen.mp4.MP4Cover(image.data, kind)
 
 
diff --git a/test/test_mediafile.py b/test/test_mediafile.py
index 8b4386f5b..c8e878086 100644
--- a/test/test_mediafile.py
+++ b/test/test_mediafile.py
@@ -46,6 +46,14 @@ class ArtTestMixin(object):
         return self._jpg_data
     _jpg_data = None
 
+    @property
+    def tiff_data(self):
+        if not self._jpg_data:
+            with open(os.path.join(_common.RSRC, 'image-2x3.tiff'), 'rb') as f:
+                self._jpg_data = f.read()
+        return self._jpg_data
+    _jpg_data = None
+
     def test_set_png_art(self):
         mediafile = self._mediafile_fixture('empty')
         mediafile.art = self.png_data
@@ -578,6 +586,13 @@ class MP4Test(ReadWriteTestBase, PartialTestMixin,
         'bitdepth': 16,
         'channels': 2,
     }
+
+    def test_add_tiff_image_fails(self):
+        mediafile = self._mediafile_fixture('empty')
+        with self.assertRaises(ValueError):
+            mediafile.images = [Image(data=self.tiff_data)]
+
+
 class AlacTest(ReadWriteTestBase, GenreListTestMixin, unittest.TestCase):
     extension = 'alac.m4a'
     audio_properties = {