Include file name in response, and support "Accept" header that contain charset property

[johan-perso]

This PR has been made to allow clients to use the API with an Accept header defined to application/json; charset=utf-8, which is especially useful for Flutter apps, as they automatically set it instead of the regular application/json.

Additionally, when requesting a download, the API will now return the name of the file. This enhancement aims to improve the developer experience when programmatically accessing Cobalt.