Create ImageField files from a callable

Allow dynamic creation of ``factory.django.ImageField`` fields
by passing a function that returns an image file object.

docs/orms.rst

@@ -143,6 +143,7 @@ Extra fields
                               and keep its filename
         :param file from_file: Use the contents of the provided file object; use its filename
                                if available
+        :param func from_func: Use function that returns a file object
         :param str filename: The filename for the ImageField
         :param int width: The width of the generated image (default: ``100``)
         :param int height: The height of the generated image (default: ``100``)

factory/django.py

@@ -205,9 +205,10 @@ def _make_data(self, params):
     def _make_content(self, params):
         path = ''
 
-        if params.get('from_path') and params.get('from_file'):
+        _content_params = [params.get('from_path'), params.get('from_file'), params.get('from_func')]
+        if len([p for p in _content_params if p]) > 1:
             raise ValueError(
-                "At most one argument from 'from_file' and 'from_path' should "
+                "At most one argument from 'from_file', 'from_path', and 'from_func' should "
                 "be non-empty when calling factory.django.FileField."
             )
 
@@ -221,6 +222,11 @@ def _make_content(self, params):
             content = django_files.File(f)
             path = content.name
 
+        elif params.get('from_func'):
+            func = params['from_func']
+            content = django_files.File(func())
+            path = content.name
+
         else:
             data = self._make_data(params)
             content = django_files.base.ContentFile(data)