webix_debug.js: 35643
Code is expecting a string object from the defined handler for the upload event, but when it encounters a proxy object instead, it fails to handle it properly
Proxy objects do not work with file uploading, this is known limitation.
Can you share a bit more info about your use-case, why the proxy is necessary for the file uploading? As far as I can see, there are not so many ways how file can be sent to a server-side.