WebP images won't upload

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

#### Current behaviour

> Screenshot (optional)

Provide steps to reproduce:

1. Go to **Insert Image**
2. Select a `.webp` image
3. Click **Open**
4. Nothing uploads

#### Expected behaviour

WebP images should upload and insert, same as JPEG and PNG images.

#### Environment

I use HackMD on:

**Desktop**
- OS: macOS
- Browser: chrome
- Browser Version: 84

**Laptop**
- OS: ChromeOS
- Browser: chrome
- Browser Version: 84