diff --git a/fileformats/core/__init__.py b/fileformats/core/__init__.py
index 9e64352..c3cf12a 100644
--- a/fileformats/core/__init__.py
+++ b/fileformats/core/__init__.py
@@ -1,7 +1,7 @@
 from ._version import __version__
 from .classifier import Classifier, ClassifierCategory
 from .datatype import DataType
-from .fileset import FileSet
+from .fileset import FileSet, MockMixin
 from .field import Field
 from .utils import (
     to_mime,
diff --git a/fileformats/core/hook.py b/fileformats/core/hook.py
index ed9752c..1bf529a 100644
--- a/fileformats/core/hook.py
+++ b/fileformats/core/hook.py
@@ -199,13 +199,13 @@ def type_match(a, b) -> bool:
             ftype = fparam.annotation
             if mname != fname:
                 differences.append(
-                    f"Name of parameter at position {i}: {mname} vs {fname}"
+                    f"name of parameter at position {i}: {mname} vs {fname}"
                 )
             elif not type_match(mtype, ftype):
                 differences.append(f"Type of '{mname}' arg: {mtype} vs {ftype}")
         if not type_match(msig.return_annotation, fsig.return_annotation):
             differences.append(
-                f"Return type: {msig.return_annotation} vs {fsig.return_annotation}"
+                f"return type: {msig.return_annotation} vs {fsig.return_annotation}"
             )
         if differences:
             raise TypeError(
diff --git a/fileformats/core/tests/test_extras.py b/fileformats/core/tests/test_extras.py
deleted file mode 100644
index eeae59c..0000000
--- a/fileformats/core/tests/test_extras.py
+++ /dev/null
@@ -1,25 +0,0 @@
-from pathlib import Path
-import platform
-from fileformats.core.fileset import MockMixin
-from fileformats.testing import Foo
-
-
-def test_sample():
-    test_inst = Foo.sample()
-    assert test_inst.fspath.exists()
-    assert test_inst.fspath.suffix == ".foo"
-
-
-def test_mock():
-    mock = Foo.mock()
-    if platform.system() == "Windows":
-        expected = Path(f"{Path().cwd().drive}\\mock\\foo.foo")
-    else:
-        expected = Path("/mock/foo.foo")
-    assert mock.fspath == expected
-    assert not mock.fspath.exists()
-    assert isinstance(mock, MockMixin)
-
-
-def test_extra_errors():
-    pass
diff --git a/fileformats/core/tests/test_hooks.py b/fileformats/core/tests/test_hooks.py
new file mode 100644
index 0000000..3b18559
--- /dev/null
+++ b/fileformats/core/tests/test_hooks.py
@@ -0,0 +1,73 @@
+from pathlib import Path
+import platform
+import typing as ty
+import pytest
+from fileformats.core import hook, MockMixin, FileSet
+from fileformats.testing import Foo
+
+
+def test_sample():
+    test_inst = Foo.sample()
+    assert test_inst.fspath.exists()
+    assert test_inst.fspath.suffix == ".foo"
+
+
+def test_mock():
+    mock = Foo.mock()
+    if platform.system() == "Windows":
+        expected = Path(f"{Path().cwd().drive}\\mock\\foo.foo")
+    else:
+        expected = Path("/mock/foo.foo")
+    assert mock.fspath == expected
+    assert not mock.fspath.exists()
+    assert isinstance(mock, MockMixin)
+
+
+class Woo(FileSet):
+    @hook.extra
+    def test_hook(self, a: int, b: float, c: ty.Optional[str] = None) -> float:
+        raise NotImplementedError
+
+
+def test_hook_signature_no_default():
+    @Woo.test_hook.register
+    def woo_test_hook(woo: Woo, a: int, b: float) -> float:
+        pass
+
+
+def test_hook_signature1():
+
+    with pytest.raises(TypeError, match="missing required argument"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(woo: Woo, a: int) -> float:
+            pass
+
+
+def test_hook_signature2():
+
+    with pytest.raises(TypeError, match="name of parameter"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(woo: Woo, a: int, d: str) -> float:
+            pass
+
+
+def test_hook_signature3():
+
+    with pytest.raises(TypeError, match="found additional argument"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(
+            woo: Woo, a: int, b: float, c: ty.Optional[str], d: int
+        ) -> float:
+            pass
+
+
+def test_hook_signature4():
+
+    with pytest.raises(TypeError, match="return type"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(woo: Woo, a: int, b: str) -> int:
+            pass