code: function (fieldname, file, filename) {
            const acceptedExtensions = strapi.api.upload.config.acceptedExtensions || [];
            if (acceptedExtensions[0] !== '*' && !_.contains(acceptedExtensions, path.extname(filename))) {
              this.status = 400;
              this.body = {
                message: 'Invalid file format ' + filename ? 'for this file' + filename : ''
              };
            }
          }
docs: Validation used by `co-busboy`.