add /files/ and /item_files/ API endpoints

backend/files/serializers.py

@@ -0,0 +1,20 @@
+from django.core.files.base import ContentFile
+from rest_framework import serializers
+
+from files.models import File
+
+
+class FileSerializer(serializers.Serializer):
+    data = serializers.CharField()
+    mime_type = serializers.CharField()
+    class Meta:
+        model = File
+        fields = ['data', 'mime_type']
+        read_only_fields = ['id', 'size', 'name']
+
+    def to_representation(self, instance):
+        return {'id': instance.id, 'name': instance.file.url, 'size': instance.file.size,
+                'mime_type': instance.mime_type}
+
+    def create(self, validated_data):
+        return File.objects.get_or_create(**validated_data)[0]