xhrupload: Fix getting upload URL

src/plugins/XHRUpload.js

@@ -77,7 +77,7 @@ module.exports = class XHRUpload extends Plugin {
       xhr.addEventListener('load', (ev) => {
         if (ev.target.status >= 200 && ev.target.status < 300) {
           const resp = opts.getResponseData(xhr)
-          const uploadURL = resp[this.responseUrlFieldName]
+          const uploadURL = resp[opts.responseUrlFieldName]
 
           this.core.emitter.emit('core:upload-success', file.id, resp, uploadURL)