Added new FileField with GridFS support

The API is similar to that of PyMongo and most of the same operations are
possible.

The FileField can be written too with put(), write() or by using the assignment
operator. All three cases are demonstrated in the tests. Metadata can be added
to a FileField by assigning keyword arguments when using put() or new_file().

tests/fields.py

@@ -3,6 +3,7 @@ import datetime
 from decimal import Decimal
 
 import pymongo
+import gridfs
 
 from mongoengine import *
 from mongoengine.connection import _get_db
@@ -607,6 +608,61 @@ class FieldTest(unittest.TestCase):
 
         Shirt.drop_collection()
 
+    def test_file_fields(self):
+        """Ensure that file fields can be written to and their data retrieved
+        """
+        class PutFile(Document):
+            file = FileField()
+
+        class StreamFile(Document):
+            file = FileField()
+
+        class SetFile(Document):
+            file = FileField()
+
+        text = 'Hello, World!'
+        more_text = 'Foo Bar'
+        content_type = 'text/plain'
+
+        PutFile.drop_collection()
+        StreamFile.drop_collection()
+        SetFile.drop_collection()
+
+        putfile = PutFile()
+        putfile.file.put(text, content_type=content_type)
+        putfile.save()
+        putfile.validate()
+        result = PutFile.objects.first()
+        self.assertTrue(putfile == result)
+        self.assertEquals(result.file.read(), text)
+        self.assertEquals(result.file.content_type, content_type)
+        result.file.delete() # Remove file from GridFS
+
+        streamfile = StreamFile()
+        streamfile.file.new_file(content_type=content_type)
+        streamfile.file.write(text)
+        streamfile.file.write(more_text)
+        streamfile.file.close()
+        streamfile.save()
+        streamfile.validate()
+        result = StreamFile.objects.first()
+        self.assertTrue(streamfile == result)
+        self.assertEquals(result.file.read(), text + more_text)
+        self.assertEquals(result.file.content_type, content_type)
+        result.file.delete() # Remove file from GridFS
+
+        setfile = SetFile()
+        setfile.file = text
+        setfile.save()
+        setfile.validate()
+        result = SetFile.objects.first()
+        self.assertTrue(setfile == result)
+        self.assertEquals(result.file.read(), text)
+        result.file.delete() # Remove file from GridFS
+
+        PutFile.drop_collection()
+        StreamFile.drop_collection()
+        SetFile.drop_collection()
 
 
 if __name__ == '__main__':