fix: Finetuning openapi doc for better swagger testing

[bestmazzo]

Description

Problem

When testing hasura-storage through Swagger UI, the POST /file endpoint doesn't allow to browse and pick a file to upload

Solution

Enriching openapi.yaml with extended property description for the endpoint solves the issue

Notes

This is a super simple fix, but I hope it may help others

[dbarrosop]

Thanks!