fix: allow a wider range of filenames

[lajp]

This PR improves on filename matching in order to e.g. render .jpeg files as parts of the pdf.
Additionally it disallows unsupported filenames entirely, returing an error if encountered.

This includes the current progress of #23 but is to be merged beforehand in order to get the hotter fixes in ASAP

[lajp]

This requires the addition of the env variable ALLOWED_ORIGINS=http://localhost:3000,https://tietokilta.fi
and the EXPOSE env var is changed to BIND_ADDR=0.0.0.0

[kahlstrm]

Looks good, just need to add the default host setting to the Dockerfile, otherwise this will not work without infra changes.