We use basic authentication to keep thins as simple as possible. You have to use your API key as username, and left password blank or enter any text of your choice, since this value won't be used.
You can get your API key from your own conversorsepa.es account.
All data sent to us or retuned by our API is in JSON format. In those cases where a file is included in the response, it comes as a strict Base64 encoded string.