How to proxy static files?

I have files in a local folder, but not always. If I have, then I transmit them. if I don't have, then I proxy request on a server where they have.

I've tried to use proxy and staticOptions, but I can't correct setting

Request:

localhost:3000/files/en/test.mo

How I've tried

proxy: {
  context: () => true,
  '/files/en': {
    target: './locale/en/',
  },
},

or

staticOptions: {
  'files/en': './locale/en/',
},