Hey @Javi_V! Thanks for reaching out.
I think @rsanders's suggestions here are right on the money, primarily because you'll need to construct the multipart/form-data payload manually and thus understand the expected format. As mentioned, this will likely require that your resource query have a raw input, instead.
It's not exactly what you're trying to do, but there's a good post here that outlines a similar issue and solution. In your case, I would start by hardcoding a valid request body in order to better understand how we need to manipulate the base64 data!