🏷️(backend) add content-type to uploaded files

All the uploaded files had the content-type set
to `application/octet-stream`. It create issues
when the file is downloaded from the frontend
because the browser doesn't know how to handle
the file.
We now determine the content-type of the file
and set it to the file object.

CHANGELOG.md

@@ -13,7 +13,7 @@ and this project adheres to
 
 - github actions to managed Crowdin workflow
 - 📈Integrate Posthog #540
-
+- 🏷️(backend) add content-type to uploaded files #552
 
 ## Changed