what to pass in filecontent in core _files upload Moodle webservice

Question

I am trying to use the core_files_upload function and the file goes in the $filecontent parameter in base64, however, in the test I passed the base64 of a .jpg image the function returns me a "File not specified" error message.

Error: { "Exception": "moodle_exception", "Errorcode": "nofile", "Message": "File not specified." }