api: allow images to be properly proxied to backend

[limwa]

          > Also, I know that it is out of the scope of this PR but won't this line (https://github.com/NIAEFEUP/website-niaefeup-frontend/pull/215/files#diff-4bcda89884a979801f8a542290893a21e5213fc48218ec305b5434ef11e70ddaR6) cause an error when the request body is, for instance, an image?

I am not sure that it would cause an error (did a quick search and the docs do not appear to mention that), maybe it will simply be a utf-8 string representation of the bytes.
I do agree that that may not be the best way to handle those cases, feel free to open an issue about that.

Originally posted by @bdmendes in #215 (comment)

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The change should be pretty simple. Instead of using the text method, we can use the arrayBuffer method and pipe that to the next fetch. Use of streams should also be investigated.