diff --git a/requirements.txt b/requirements.txt
index 290cd98..62be83b 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,7 @@
 # Optionals
 jinja2
 pyyaml
+puremagic
 
 # Packaging
 twine
diff --git a/setup.py b/setup.py
index ebc4b01..a852a74 100755
--- a/setup.py
+++ b/setup.py
@@ -44,6 +44,7 @@ def get_packages(package):
     extras_require={
         "jinja2": ["jinja2"],
         "pyyaml": ["pyyaml"],
+        "puremagic": ["puremagic"],
     },
     python_requires='>=3.6',
     classifiers=[
diff --git a/tests/image/head.jpeg b/tests/image/head.jpeg
new file mode 100644
index 0000000..aa8d72c
Binary files /dev/null and b/tests/image/head.jpeg differ
diff --git a/tests/test_fields.py b/tests/test_fields.py
index 3b0fb49..e96961b 100644
--- a/tests/test_fields.py
+++ b/tests/test_fields.py
@@ -16,7 +16,9 @@
     DateTime,
     Decimal,
     Email,
+    File,
     Float,
+    Image,
     Integer,
     IPAddress,
     Number,
@@ -889,3 +891,40 @@ def test_url():
     validator = URL()
     value, error = validator.validate_or_error("example")
     assert error == ValidationError(text="Must be a real URL.", code="invalid")
+
+
+def test_file():
+    validator = File()
+    with open("test.txt", "w") as f:
+        f.write("123")
+    with open("test.txt") as f:
+        value, error = validator.validate_or_error(f)
+        assert value == f
+
+    validator = File()
+    value, error = validator.validate_or_error(None)
+    assert error == ValidationError(text="Must be a file descriptor.", code="type")
+
+
+def test_image():
+    validator = Image(image_types=["png"])
+    value, error = validator.validate_or_error(None)
+    assert error == ValidationError(text="Must be a file descriptor.", code="type")
+
+    with open("test.png", "wb") as f:
+        f.write(b"\211PNG\r\n\032\nxxxxxxxxxxxxxxxxxxxxxxxy")
+
+    with open("test.png", "rb") as f:
+        validator = Image(image_types=["png"])
+        value, error = validator.validate_or_error(f)
+        assert error == ValidationError(
+            text="Do not support this image type.", code="image_types"
+        )
+        validator = Image()
+        value, error = validator.validate_or_error(f)
+        assert value is f
+
+    with open("./tests/image/head.jpeg", "rb") as f:
+        validator = Image(image_types=["jfif"])
+        value, error = validator.validate_or_error(f)
+        assert value == f
diff --git a/typesystem/fields.py b/typesystem/fields.py
index 2d47d5a..9562e82 100644
--- a/typesystem/fields.py
+++ b/typesystem/fields.py
@@ -1,4 +1,5 @@
 import decimal
+import io
 import re
 import typing
 from math import isfinite
@@ -7,6 +8,13 @@
 from typesystem.base import Message, ValidationError, ValidationResult
 from typesystem.unique import Uniqueness
 
+try:
+    # check the image type.
+    import puremagic
+except ImportError:
+    puremagic = None
+
+
 NO_DEFAULT = object()
 
 FORMATS = {
@@ -385,7 +393,7 @@ def validate(self, value: typing.Any) -> typing.Any:
             return None
         elif value is None:
             raise self.validation_error("null")
-        elif value not in Uniqueness([key for key, value in self.choices]):
+        elif value not in Uniqueness([key for key, val in self.choices]):
             if value == "":
                 if self.allow_null and self.coerce_types:
                     return None
@@ -749,7 +757,7 @@ def validate(self, value: typing.Any) -> typing.Any:
 
 class Const(Field):
     """
-    Only ever matches the given given value.
+    Only ever matches the given value.
     """
 
     errors = {"only_null": "Must be null.", "const": "Must be the value '{const}'."}
@@ -790,3 +798,53 @@ def __init__(self, **kwargs: typing.Any) -> None:
 class URL(String):
     def __init__(self, **kwargs: typing.Any) -> None:
         super().__init__(format="url", **kwargs)
+
+
+class File(Field):
+    errors = {
+        "type": "Must be a file descriptor.",
+    }
+    value_types = (
+        io.BufferedReader,
+        io.TextIOWrapper,
+        io.BufferedRandom,
+        io.BufferedWriter,
+    )
+
+    def __init__(self, **kwargs: typing.Any) -> None:
+        super().__init__(**kwargs)
+
+    def validate(self, value: typing.Any) -> typing.Any:
+        if not isinstance(value, self.value_types):
+            raise self.validation_error("type")
+        return value
+
+
+class Image(File):
+    errors = {
+        "type": "Must be a file descriptor.",
+        "image_types": "Do not support this image type.",
+    }
+
+    def __init__(
+        self, image_types: typing.List[str] = None, **kwargs: typing.Any
+    ) -> None:
+
+        super().__init__(**kwargs)
+        self.image_types = image_types
+
+    def validate(self, value: typing.Any) -> typing.Any:
+        value = super().validate(value)
+        if self.image_types is None:
+            return value
+
+        assert puremagic is not None, "'puremagic' must be installed."
+        try:
+            image_type: typing.Optional[str] = puremagic.from_stream(value)
+        except Exception:
+            image_type = None
+
+        image_type = (image_type or "").strip(".")
+        if not image_type or image_type not in self.image_types:
+            raise self.validation_error("image_types")
+        return value
diff --git a/typesystem/json_schema.py b/typesystem/json_schema.py
index b03135e..321b20f 100644
--- a/typesystem/json_schema.py
+++ b/typesystem/json_schema.py
@@ -408,7 +408,7 @@ def to_json_schema(
     elif isinstance(arg, NeverMatch):
         return False
 
-    field: typing.Optional[Field]
+    field: typing.Optional[Field] = None
     data: dict = {}
     is_root = _definitions is None
     definitions = {} if _definitions is None else _definitions
@@ -416,7 +416,6 @@ def to_json_schema(
     if isinstance(arg, Field):
         field = arg
     elif isinstance(arg, Definitions):
-        field = None
         for key, value in arg.items():
             definitions[key] = to_json_schema(value, _definitions=definitions)