✨(backend) add file upload feature & tests

For the coming features we will need to store files on the meet side.
(for instance user backgrounds).

This commits adds a new Model to manage files, and the associated
serializers & viewsets. All are tested.

This work was heavily inspired by the work done by our friends at
https://github.com/suitenumerique/drive
It build on the same architecture design (upload directly to S3 but
download goes through our proxy), but model is much much simplier
(no folders, no file sharing, etc.).

src/backend/core/factories.py

@@ -2,8 +2,11 @@
 Core application factories
 """
 
+from io import BytesIO
+
 from django.conf import settings
 from django.contrib.auth.hashers import make_password
+from django.core.files.storage import default_storage
 from django.utils.text import slugify
 
 import factory.fuzzy
@@ -153,3 +156,42 @@ class ApplicationDomainFactory(factory.django.DjangoModelFactory):
 
     domain = factory.Faker("domain_name")
     application = factory.SubFactory(ApplicationFactory)
+
+
+class FileFactory(factory.django.DjangoModelFactory):
+    """A factory to create files"""
+
+    class Meta:
+        model = models.File
+        skip_postgeneration_save = True
+
+    title = factory.Sequence(lambda n: f"file{n}")
+    creator = factory.SubFactory(UserFactory)
+    deleted_at = None
+    type = factory.fuzzy.FuzzyChoice([t[0] for t in models.FileTypeChoices.choices])
+    filename = factory.lazy_attribute(lambda o: fake.file_name())
+    upload_state = None
+    size = None
+
+    @factory.post_generation
+    def update_upload_state(self, create, extracted, **kwargs):
+        """Change the upload state of a file."""
+        if create and extracted:
+            self.upload_state = extracted
+            self.save()
+
+    @factory.post_generation
+    def upload_bytes(self, create, extracted, **kwargs):
+        """Save content of the file into the storage"""
+        if create and extracted is not None:
+            content = (
+                extracted
+                if isinstance(extracted, bytes)
+                else str(extracted).encode("utf-8")
+            )
+
+            self.filename = kwargs.get("filename", self.filename or "content.txt")
+            self.size = len(content)
+            self.save()
+
+            default_storage.save(self.file_key, BytesIO(content))