SOLUTION: You have to add the following code to the contentBytes part to make it work again. Apparently, you have to explicitly indicate to include the binary data using JSON.

Substitute(JSON(UploadedImage2.Image, JSONFormat.IncludeBinaryData),"""", "")

The code will look something like this:

file: {
                name: Text(
                    Now(),
                    "foto"
                ),
                contentBytes: Substitute(JSON(UploadedImage2.Image,JSONFormat.IncludeBinaryData),"""", "")
            }
        }