file validators correctly identify empty data

closes #276

- `FileField` is no longer deprecated. The data is checked during
  processing and only set if it's a valid file.
- `has_file` *is* deprecated; it's now equivalent to
  `bool(field.data).
- `FileRequired` and `FileAllowed` work with both the Flask-WTF and
  WTForms `FileField` classes.
- The `Optional` validator now works with `FileField`.

docs/changelog.rst

@@ -1,6 +1,22 @@
 Flask-WTF Changelog
 ===================
 
+Version 0.14.1
+--------------
+
+- Fix bug where the file validators would incorrectly identify an empty file as
+  valid data. (`#276`_, `#277`_)
+
+    - ``FileField`` is no longer deprecated. The data is checked during
+      processing and only set if it's a valid file.
+    - ``has_file`` *is* deprecated; it's now equivalent to ``bool(field.data)``.
+    - ``FileRequired`` and ``FileAllowed`` work with both the Flask-WTF and
+      WTForms ``FileField`` classes.
+    - The ``Optional`` validator now works with ``FileField``.
+
+.. _#276: https://github.com/lepture/flask-wtf/issues/276
+.. _#277: https://github.com/lepture/flask-wtf/pull/277
+
 Version 0.14
 ------------
 

flask_wtf/file.py

@@ -1,34 +1,34 @@
 import warnings
-
 from collections import Iterable
+
 from werkzeug.datastructures import FileStorage
 from wtforms import FileField as _FileField
 from wtforms.validators import DataRequired, StopValidation
 
-from flask_wtf._compat import FlaskWTFDeprecationWarning
+from ._compat import FlaskWTFDeprecationWarning
 
 
 class FileField(_FileField):
-    """
-    Werkzeug-aware subclass of **wtforms.FileField**
+    """Werkzeug-aware subclass of **wtforms.FileField**"""
 
-    .. deprecated:: 0.14
-        ``has_file`` was simplified and merged into the validators.
-        This subclass is no longer needed and will be removed in 1.0.
-    """
+    def process_formdata(self, valuelist):
+        valuelist = (x for x in valuelist if isinstance(x, FileStorage) and x)
+        data = next(valuelist, None)
 
-    def __new__(cls, *args, **kwargs):
-        warnings.warn(FlaskWTFDeprecationWarning(
-            'The "FileField" subclass is no longer necessary and will be '
-            'removed in 1.0. Use "wtforms.FileField" directly instead.'
-        ), stacklevel=2)
-        return super(FileField, cls).__new__(cls, *args, **kwargs)
+        if data is not None:
+            self.data = data
+        else:
+            self.raw_data = ()
 
     def has_file(self):
-        """Return True if self.data is a
+        """Return ``True`` if ``self.data`` is a
         :class:`~werkzeug.datastructures.FileStorage` object."""
 
-        return isinstance(self.data, FileStorage)
+        warnings.warn(FlaskWTFDeprecationWarning(
+            '"has_file" is deprecated and will be removed in 1.0. The data is '
+            'checked during processing instead.'
+        ))
+        return bool(self.data)
 
 
 class FileRequired(DataRequired):
@@ -41,7 +41,7 @@ class FileRequired(DataRequired):
     """
 
     def __call__(self, form, field):
-        if not isinstance(field.data, FileStorage):
+        if not (isinstance(field.data, FileStorage) and field.data):
             if self.message is None:
                 message = field.gettext('This field is required.')
             else:
@@ -68,7 +68,7 @@ def __init__(self, upload_set, message=None):
         self.message = message
 
     def __call__(self, form, field):
-        if not isinstance(field.data, FileStorage):
+        if not (isinstance(field.data, FileStorage) and field.data):
             return
 
         filename = field.data.filename.lower()