Ok, so curl is ok now, the problem is upload from a local file, I proved using the path of the file as a sting and the file content as you see in the code, but any of both worked. I get the message:

Array ( [error] => Array ( [code] => missingparam [info] => One of the parameters sessionkey, file, url is required ) )

What's wrong then? (I have no errors in error.log now).