Use bpp from ICO header

Tests/test_file_ico.py

@@ -18,6 +18,12 @@ def test_sanity():
     assert im.get_format_mimetype() == "image/x-icon"
 
 
+def test_black_and_white():
+    with Image.open("Tests/images/black_and_white.ico") as im:
+        assert im.mode == "RGBA"
+        assert im.size == (16, 16)
+
+
 def test_invalid_file():
     with open("Tests/images/flower.jpg", "rb") as fp:
         with pytest.raises(SyntaxError):

src/PIL/IcoImagePlugin.py

@@ -190,13 +190,7 @@ def frame(self, idx):
             im.tile[0] = d, (0, 0) + im.size, o, a
 
             # figure out where AND mask image starts
-            mode = a[0]
-            bpp = 8
-            for k, v in BmpImagePlugin.BIT2MODE.items():
-                if mode == v[1]:
-                    bpp = k
-                    break
-
+            bpp = header["bpp"]
             if 32 == bpp:
                 # 32-bit color depth icon image allows semitransparent areas
                 # PIL's DIB format ignores transparency bits, recover them.