✨(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/utils.py

@@ -19,6 +19,7 @@ from django.conf import settings
 from django.core.files.storage import default_storage
 
 import aiohttp
+import boto3
 import botocore
 import magic
 from asgiref.sync import async_to_sync
@@ -382,8 +383,10 @@ def detect_mimetype(file_buffer: bytes, filename: str | None = None) -> str:
         # Use guess_file_type (Python 3.13+) instead of deprecated guess_type
         mimetype_from_extension, _ = mimetypes.guess_file_type(filename, strict=False)
 
-    logger.info("detect_mimetype: mimetype_from_content: %s", mimetype_from_content)
-    logger.info("detect_mimetype: mimetype_from_extension: %s", mimetype_from_extension)
+    logger.debug("detect_mimetype: mimetype_from_content: %s", mimetype_from_content)
+    logger.debug(
+        "detect_mimetype: mimetype_from_extension: %s", mimetype_from_extension
+    )
 
     # Strategy: Prefer content-based detection, but use extension if:
     # 1. Content detection returns generic types (application/octet-stream, text/plain)
@@ -411,3 +414,44 @@ def detect_mimetype(file_buffer: bytes, filename: str | None = None) -> str:
 
     # Default to content-based detection (most reliable)
     return mimetype_from_content or "application/octet-stream"
+
+
+def generate_upload_policy(file):
+    """
+    Generate a S3 upload policy for a given file.
+
+    Notes:
+        Originally taken from https://github.com/suitenumerique/drive/blob/564822d31f071c6dfacd112ef4b7146c73077cd9/src/backend/core/api/utils.py#L102  # pylint: disable=line-too-long
+    """
+
+    key = file.file_key
+
+    # This settings should be used if the backend application and the frontend application
+    # can't connect to the object storage with the same domain. This is the case in the
+    # docker compose stack used in development. The frontend application will use localhost
+    # to connect to the object storage while the backend application will use the object storage
+    # service name declared in the docker compose stack.
+    # This is needed because the domain name is used to compute the signature. So it can't be
+    # changed dynamically by the frontend application.
+    if settings.AWS_S3_DOMAIN_REPLACE:
+        s3_client = boto3.client(
+            "s3",
+            aws_access_key_id=settings.AWS_S3_ACCESS_KEY_ID,
+            aws_secret_access_key=settings.AWS_S3_SECRET_ACCESS_KEY,
+            endpoint_url=settings.AWS_S3_DOMAIN_REPLACE,
+            config=botocore.client.Config(
+                region_name=settings.AWS_S3_REGION_NAME,
+                signature_version=settings.AWS_S3_SIGNATURE_VERSION,
+            ),
+        )
+    else:
+        s3_client = default_storage.connection.meta.client
+
+    # Generate the policy
+    policy = s3_client.generate_presigned_url(
+        ClientMethod="put_object",
+        Params={"Bucket": default_storage.bucket_name, "Key": key, "ACL": "private"},
+        ExpiresIn=settings.AWS_S3_UPLOAD_POLICY_EXPIRATION,
+    )
+
+    return policy