Working project

Proof/build/electron-publisher-custom.js

@@ -0,0 +1,15 @@
+const { Publisher } = require("electron-publish");
+const fsPromises = require("fs/promises");
+const path = require("path");
+
+module.exports = class CustomPublisher extends Publisher {
+  // more about task:
+  //   https://github.com/electron-userland/electron-builder/blob/a94532164709a545c0f6551fdc336dbc5377bda8/packages/electron-publish/src/publisher.ts#L29
+  async upload(task) {
+    const filename = path.basename(task.file);
+    await fsPromises.cp(
+      task.file,
+      path.join(__dirname, "../../publish", filename)
+    );
+  }
+};