MIME Based History Fallback Middleware

server.js

import express from 'express'
import path from 'path'
import mime from 'mime'

const app = express()
app.use(express.static(buildPath))
app.use((req, res, next) => {
  if (req.method === 'GET' && mime.lookup(req.path) === mime.default_type) {
    res.sendFile('index.html')
    return
  }

  next()  
})

Tested in https://github.com/unfold/avinor-next/commit/bc73f9faca19e80b944a83b1c7edc22717699a36 and works as expected