diff --git a/nibabel/freesurfer/mghformat.py b/nibabel/freesurfer/mghformat.py
index f63875b2c6..1091e256a9 100644
--- a/nibabel/freesurfer/mghformat.py
+++ b/nibabel/freesurfer/mghformat.py
@@ -454,13 +454,13 @@ def writeftr_to(self, fileobj):
         fileobj.write(ftr_nd.tostring())
 
 
-# Register .mgz extension as compressed
-@ImageOpener.register_ext_from_image('.mgz', ImageOpener.gz_def)
 class MGHImage(SpatialImage):
     """ Class for MGH format image
     """
     header_class = MGHHeader
-    valid_exts = ('.mgh',)
+    valid_exts = ('.mgh', '.mgz')
+    # Register that .mgz extension signals gzip compression
+    ImageOpener.compress_ext_map['.mgz'] = ImageOpener.gz_def
     files_types = (('image', '.mgh'),)
     _compressed_suffixes = ()
 
diff --git a/nibabel/openers.py b/nibabel/openers.py
index 26bbb09d7b..1969766a55 100644
--- a/nibabel/openers.py
+++ b/nibabel/openers.py
@@ -149,42 +149,22 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
 
 class ImageOpener(Opener):
-    """ Opener-type class passed to image classes to collect compressed extensions
+    """ Opener-type class to collect extra compressed extensions
 
-    This class allows itself to have image extensions added to its class
-    attributes, via the `register_ex_from_images`.  The class can therefore
-    change state when image classes are defined.
+    A trivial sub-class of opener to which image classes can add extra
+    extensions with custom openers, such as compressed openers.
+
+    To add an extension, add a line to the class definition (not __init__):
+
+        ImageOpener.compress_ext_map[ext] = func_def
+
+    ``ext`` is a file extension beginning with '.' and should be included in
+    the image class's ``valid_exts`` tuple.
+
+    ``func_def`` is a `(function, (args,))` tuple, where `function accepts a
+    filename as the first parameter, and `args` defines the other arguments
+    that `function` accepts. These arguments must be any (unordered) subset of
+    `mode`, `compresslevel`, and `buffering`.
     """
+    # Add new extensions to this dictionary
     compress_ext_map = Opener.compress_ext_map.copy()
-
-    @classmethod
-    def register_ext_from_image(opener_klass, ext, func_def):
-        """Decorator for adding extension / opener_function associations.
-
-        Should be used to decorate classes. Updates ImageOpener class with
-        desired extension / opener association. Updates decorated class by
-        adding ```ext``` to ```klass.alternate_exts```.
-
-        Parameters
-        ----------
-        opener_klass : decorated class
-        ext : file extension to associate `func_def` with.
-          should start with '.'
-        func_def : opener function/parameter tuple
-          Should be a `(function, (args,))` tuple, where `function` accepts
-          a filename as the first parameter, and `args` defines the
-          other arguments that `function` accepts. These arguments must
-          be any (unordered) subset of `mode`, `compresslevel`,
-          and `buffering`.
-
-        Returns
-        -------
-        opener_klass
-        """
-        def decorate(klass):
-            assert ext not in opener_klass.compress_ext_map, \
-                "Cannot redefine extension-function mappings."
-            opener_klass.compress_ext_map[ext] = func_def
-            klass.valid_exts += (ext,)
-            return klass
-        return decorate
diff --git a/nibabel/tests/test_openers.py b/nibabel/tests/test_openers.py
index 08f9730ace..2900a0437e 100644
--- a/nibabel/tests/test_openers.py
+++ b/nibabel/tests/test_openers.py
@@ -93,8 +93,6 @@ def test_BinOpener():
 
 
 class TestImageOpener:
-    valid_exts = ()
-
     def setUp(self):
         self.compress_ext_map = ImageOpener.compress_ext_map.copy()
 
@@ -115,12 +113,9 @@ def file_opener(fileish, mode):
 
         # Add the association
         n_associations = len(ImageOpener.compress_ext_map)
-        dec = ImageOpener.register_ext_from_image('.foo',
-                                                  (file_opener, ('mode',)))
-        dec(self.__class__)
+        ImageOpener.compress_ext_map['.foo'] = (file_opener, ('mode',))
         assert_equal(n_associations + 1, len(ImageOpener.compress_ext_map))
         assert_true('.foo' in ImageOpener.compress_ext_map)
-        assert_true('.foo' in self.valid_exts)
 
         with InTemporaryDirectory():
             with ImageOpener('test.foo', 'w'):