Register transforms for FileField. Fix #60

It turns out the existing transformations were not registered.

Also add a test for #60.

Author: atodorov

pylint_django/tests/input/func_noerror_model_fields.py

@@ -76,7 +76,9 @@ def durationfield_tests(self):
         print(self.durationfield.total_seconds())
 
     def filefield_tests(self):
+        self.filefield.save('/dev/null', 'TEST')
         print(self.filefield.file)
+        self.imagefield.save('/dev/null', 'TEST')
         print(self.imagefield.file)
 
     def numberfield_tests(self):

pylint_django/transforms/__init__.py

@@ -46,3 +46,5 @@ def set_fake_locals(module):
                'Manager')
 _add_transform('django.utils.translation', 'ugettext_lazy')
 _add_transform('mongoengine', 'Document')
+# register transform for FileField/ImageField, see #60
+_add_transform('django.db.models.fields.files', 'FileField')