Problem loading images to gitlab with `_` in the name

I was debugging a problem I found with images not showing up in gitlab when using `cml comment create` and found some strange things, here is my test set:

```
- echo "# no changes" > report.md
- echo "" >> report.md
- cp ./reports/model_pr_curves/person_iou0.5_51.png test.png
- echo "# local_no_special_chars" >> report.md
- echo "" >> report.md
- cp ./reports/model_pr_curves/person_iou0.5_51.png ./reports/model_pr_curves/test.png
- echo "# nested_no_special_chars" >> report.md
- echo "" >> report.md
- cp ./reports/model_pr_curves/person_iou0.5_51.png ./d_._test.png
- echo "# local_special_chars" >> report.md
- echo "" >> report.md
- cp ./reports/model_pr_curves/person_iou0.5_51.png ./reports/model_pr_curves/aaaaaaaaaaaaaaaa.png
- echo "# long_path" >> report.md
- echo "" >> report.md
- echo "" >> report.md
- npx github:iterative/cml#warn-publish-enoent comment create --publish-native report.md

no changes (atest) fails to load, local_no_special_chars (btest) loads, nested_no_special_chars (ctest) loads, local_special_chars (dtest) fails to load, and long_path (etest) loads
```

It appears that `cml` doesn't like the `_` character in the name. Also I noticed that on a different run the long_path test failed to load, I wonder if the content in the square brackets needs to be unique?