Can can I use FILES["source"] value in the POST request to upload images from the hard drive (local images) using the API?
No matter how I try to modify the source value, I still get "Invalid base64 string." error message in response.
Yes, you can do it. But you have to send the request as a multipart/form-data. Since you aren't sending the request as multipart, the system assumes that the source is a string (base64). You either forge the form or you POST a base64 image.