🔒️(back) set ContentDisposition on media upload

On the media upload endpoint, we want to set the content-disposition
header. Its value is based on the uploaded file mime-type and if flagged
as unsafe. If the file is not an image or is unsafe then the
contentDisposition is set to attachment to force its download.
Otherwise, we set it to inline.

src/backend/core/api/serializers.py

@@ -418,6 +418,7 @@ class FileUploadSerializer(serializers.Serializer):
 
         self.context["expected_extension"] = extension
         self.context["content_type"] = magic_mime_type
+        self.context["file_name"] = file.name
 
         return file
 
@@ -426,6 +427,7 @@ class FileUploadSerializer(serializers.Serializer):
         attrs["expected_extension"] = self.context["expected_extension"]
         attrs["is_unsafe"] = self.context["is_unsafe"]
         attrs["content_type"] = self.context["content_type"]
+        attrs["file_name"] = self.context["file_name"]
         return attrs