Don't double file extension in clean_filename (#3942)

* Don't double file extension in clean_filename

* Don't use display_name as ClearableBasenameFileInput.FakeFile.name

Reason: it's used as the thumbnail source and therefore needs to be a valid file name and not some display name

src/pretix/control/forms/__init__.py

@@ -123,8 +123,6 @@ class ClearableBasenameFileInput(forms.ClearableFileInput):
 
         @property
         def name(self):
-            if hasattr(self.file, 'display_name'):
-                return self.file.display_name
             return self.file.name
 
         @property