AssertionError: missing attachment in notebook

[jtpio]

Description

It looks like pytest-check-links errors on notebook files with attachments.

Reproduce

Using this test notebook:

{
 "cells": [
  {
   "attachments": {
    "image.png": {
     "image/png": "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"
    }
   },
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Attachment:\n",
    "![image.png](attachment:image.png)"
   ]
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.6.7"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}

That renders like this in JupyterLab:

Running pytest-check-links example.ipynb gives the following error:

===================================================================================================== short test summary info =====================================================================================================
ERROR example.ipynb - AssertionError: missing attachment: image.png
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
================================================================================================== 2 warnings, 1 error in 0.31s ===================================================================================================

Expected behavior

The check should pass.

Context

Haven't looked more in details, but maybe this is more of a nbconvert issue since the assert is raised here? https://github.com/jupyter/nbconvert/blob/90b3911ed9b155e8ca3680ca4581d9344aa10be0/nbconvert/filters/markdown_mistune.py#L163

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