file:///// links skipped in HTML output of Sphinx

[Mershl]

Describe the problem

The behaviour of an inline link to an external resource (access through file://///) does not work as expected for an .md input to MyST when generating in HTML through Sphinx. (e.g. for referencing files on network shares)

Given an inline link like:

[Click me](file://///foo.com/dataXYZ/documentation.pdf)

Results in the inline link being output unprocessed:

Creating the link manually in the built html using:

<a class="reference external" href="file://///foo.com/dataXYZ/documentation.pdf">Click me</a>

Results in the expected link and works as expected in our tested browsers.

I guess that file:///// links are disabled/ignored by MyST per default as the protocol is disabled by most modern web browsers. In companies this legacy feature often times still used and enabled for the used browser. Is it possible to enable MyST to generate the expected link for this input?

Steps to reproduce

Using Sphinx and MyST.

1. Create an .md containing: [Click me](file://///foo.com/dataXYZ/documentation.pdf)
2. Add the .md to your index.rst
3. Call ./make html to start the HTML generation of Sphinx
4. Open generated .html. The file will contain the link unprocessed, looking like this:

The version of Python you're using

3.9.7

Your operating system

Windows 10

Versions of your packages

Sphinx==4.0.2 myst-parser==0.15.1 sphinx-rtd-theme==0.5.2

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[chrisjsewell]

Heya, yeh unfortunately this URL is not recognised by default in CommonMark, see e.g. this example It might be difficult to override this behaviour

perhaps overriding: https://github.com/executablebooks/markdown-it-py/blob/cde6906489c570fd3912ed0a1a6e21ae058348f7/markdown_it/main.py#L313, but I'm not definite

[choldgraf]

If you primarily need this in Sphinx, I think the download role could do the trick for you. Want to try that? https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#referencing-downloadable-files