Can't insert mages

### 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

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem
I don't seem able to insert images/pictures
#### Current behaviour
The image doesn't get uploaded.
> Screenshot (optional)

Provide steps to reproduce:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

If possible, create a note with reproducible content and share the link with us.

e.g. note link: https://hackmd.io/features

#### Expected behaviour

(description of expected behaviour)

#### Environment

I use HackMD on:

**Desktop**
- mac
- Browser

**Mobile**

- Device: Android or Mac

#### Additional context

Add any other context about the problem here.