🏷️(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.

Dockerfile

@@ -76,6 +76,8 @@ RUN apk add \
   pango \
   shared-mime-info
 
+RUN wget https://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types -O /etc/mime.types
+
 # Copy entrypoint
 COPY ./docker/files/usr/local/bin/entrypoint /usr/local/bin/entrypoint