title: django get_available_name() got an unexpected keyword argument 'max_length'
body: <p>I want to override files with the my django Model. So if I upload 'one' and later upload 'two', 'two' should override 'one' (on the file system). But I get an error.</p>
  
  <p>This is my model:</p>
  
  <pre><code>class StudentAssignment(models.Model):
      file0 = models.FileField(upload_to=use_solution_path, storage=OverwriteStorage(), validators=[validate_file_extension])
  </code></pre>
  
  <p>This is the storage.</p>
  
  <pre><code>import os
  from django.conf import settings
  from django.core.files.storage import FileSystemStorage
  
  
  class OverwriteStorage(FileSystemStorage):
      def get_available_name(self, name):
          """
          Returns a filename that's free on the target storage system, and
          available for new content to be written to.
          """
          # If the filename already exists, remove it as if it was a true file system
          if self.exists(name):
              os.remove(os.path.join(settings.MEDIA_ROOT, name))
          return name
  </code></pre>
  
  <p>The Error:</p>
  
  <pre><code>  [...]
    File "/home/mb/.local/lib/python3.5/site-packages/django/db/models/fields/files.py", line 95, in save
  self.name = self.storage.save(name, content, max_length=self.field.max_length)
    File "/home/mb/.local/lib/python3.5/site-packages/django/core/files/storage.py", line 53, in save
  name = self.get_available_name(name, max_length=max_length)
  TypeError: get_available_name() got an unexpected keyword argument 'max_length'
  </code></pre>
  
  <p>I am new to django and have no idea how to go on. Can someone help? Thanks :)</p>