`default-image-extension` should be better documented (was: A badge with no image extension is assumed to be PNG)

[maelle]

Bug description

In a blog where one of the post has this badge

[![OpenSSF Best Practices](https://bestpractices.coreinfrastructure.org/projects/1882/badge)](https://bestpractices.coreinfrastructure.org/projects/1882)

the badge is converted to

<p><a href="https://bestpractices.coreinfrastructure.org/projects/1882"><img src="https://bestpractices.coreinfrastructure.org/projects/1882/badge.png" class="img-fluid figure-img"></a></p> 

therefore the image is missing.

Interestingly in the post listing (where I would not expect it to be included, other issue #6091), the badge is

<a href="./posts/post-with-code/index.html"> <img src="https://bestpractices.coreinfrastructure.org/projects/1882/badge" class="thumnail-image"> </a>

It might unusual for a badge to be specified without extension, though. I see it's also done for metacran badges https://github.com/r-lib/usethis/blob/d3614c701b9e2ee6f680a245d3315b1b941f49de/README.Rmd#L21

Steps to reproduce

https://github.com/maelle/badgeblog/blob/main/posts/post-with-code/index.qmd#L8

Expected behavior

I would expect no extension to be added to the image in the badge.

Actual behavior

A PNG extension is added.

Your environment

RStudio 2023.03.0+386 "Cherry Blossom" Release (3c53477afb13ab959aeb5b34df1f10c237b256c3, 2023-03-09) for Ubuntu Bionic
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) rstudio/2023.03.0+386 Chrome/108.0.5359.179 Electron/22.0.3 Safari/537.36

Quarto check output

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 2.19.2: OK
      Dart Sass version 1.32.8: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.3.49
      Path: /opt/quarto/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.8.10
      Path: /usr/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.2.0
      Path: /opt/R/4.2.0/lib/R
      LibPaths:
        - /home/maelle/R/x86_64-pc-linux-gnu-library/4.2
        - /opt/R/4.2.0/lib/R/library
      rmarkdown: 2.22

[✓] Checking Knitr engine render......OK

[mcanouil]

It appears to be done by Quarto during pre-processing, as Pandoc does not have the issue.
Thanks for the report.

The fact that extension is appended is likely to break many images API such as badges.

[cscheid]

The problem here is that we use default-image-extension: png by default as a reader option.

This option is ultimately defined in src/format/formats-shared.ts:createHtmlFormat, which defines the object that holds the option which gets sent to pandoc. You can override this as such:

---
format: html
title: issue-6092
default-image-extension: ""
---

[![OpenSSF Best Practices](https://bestpractices.coreinfrastructure.org/projects/1882/badge)](https://bestpractices.coreinfrastructure.org/projects/1882)

This way you'll see that the image now has no extension. I'd like to be able to do this more cleanly but, in the meantime, that hack should work for you.

[maelle]

thank you!!

[brunj7]

I had a similar problem (with ![CERN computing center](https://cds.cern.ch/images/OPEN-PHO-CCC-2017-001-1/file?size=medium) not rendering. I am glad I found this GitHub issue, this solved my problem as well, thank you!!

My feedback (take it or leave it 🙂, especially since I do not know the motivation behind this choice) would be to encourage you to reconsider this default behavior. IMHO the image should break if the provided path is incomplete / missing the extension. This would be an easier thing to debug than an URL "automatically" being altered.