jupyter: Concurrent file serving is broken?

I’ve filled this here but as it seems bug report of that repository is not that active and my previous bug didn’t get any response I feel I should report it here also,

(rest is copied from https://github.com/jupyter/jupyter_server/issues/11, feel free close one of the files)

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I am experiencing a very weird issue with jupyter that possibly is a regression. Here is its steps to reproduce:

Create a notebook, put this code on first cell to create test images:

from PIL import Image
from PIL import ImageDraw

for i in range(1, 21):
    img = Image.new('RGBA', (10, 10), 255)
    draw = ImageDraw.Draw(img)
    draw.text((0, 0), str(i), fill="black")
    img.save(str(i) + '.png')

Put this on the next cell as markdown:

<img src="1.png" width="30">
<img src="2.png" width="30">
<img src="3.png" width="30">
<img src="4.png" width="30">
<img src="5.png" width="30">
<img src="6.png" width="30">
<img src="7.png" width="30">
<img src="8.png" width="30">
<img src="9.png" width="30">
<img src="10.png" width="30">
<img src="11.png" width="30">
<img src="12.png" width="30">
<img src="13.png" width="30">
<img src="14.png" width="30">
<img src="15.png" width="30">
<img src="17.png" width="30">
<img src="18.png" width="30">
<img src="19.png" width="30">
<img src="20.png" width="30">

Actual: I see such result this, differently every time:

Expected: To images loaded correctly.

My system: jupyter --version: 4.3.0 macOS High Sierra

I can reproduce this very consistently on my setup, Is jupyter server somehow fails on serving concurrent requests or something else is wrong?