[NGO Registration] The statute loses its extension

**Describe the bug**

The statute of the organization uploaded in the app loses its extension.

**Steps to reproduce the behavior**

1. Register an organization and add a statute
2. Go to `/organization/legal` and download the file
3. The file is downloaded without any extension

**Expected behavior**

The file's extension should be kept, especially if someone manages to upload something that isn't a PDF.

**Additional context**

We saw that some NGOs managed to upload a non-PDF file as a statute (an organization managed to upload a TXT file a few days ago).
While we weren't able to reproduce the issue, not knowing what kind of file this is could cause big problem for our workflow.