[Bug] Cannot list multi-part extension files

[Nic128]

- Do you want to request a feature or report a bug?
Bug

- What is the current behavior?
You can't list files using a multi-part extension.

- If the current behavior is a bug, please provide the steps to reproduce.

• Forked the one-click-hugo-cms: https://gitlab.com/Nic128/one-click-hugo-cms/
• Rename the posts from .md to .en.md
• Set the extension for the posts and format to:

extension: 'en.md'
format: 'frontmatter'

• Create a new post, the post will be created properly. {{slug}}.en.md
• You can't see any file listed for editing, even if the files exist.
• I see the list data from ajax calls, but the filtering in the cms code is filtering everything out.

- What is the expected behavior?
If just using md list the files correctly, en.md should list the same files.

I found out how to use multi-part extension from this thread. #1399

- Please mention your versions where applicable.

Netlify CMS version: 1.9.2
Browser version: Firefox 60

Node.JS version: 8.11.2
Operating System: OSX

- Please link or paste your config.yml below if applicable.
https://gitlab.com/Nic128/one-click-hugo-cms/blob/master/site/static/admin/config.yml

[Nic128]

I did some digging.

entriesByFolder(collection, extension) {
    return this.api.listFiles(collection.get("folder"))
    .then(files => files.filter(file => fileExtension(file.name) === extension))
    .then(this.fetchFiles);
  }

fileExtension(file.name) is made to only return md from the filename. Makes sense.

fileExtensionWithSeparator from the pathHelper.js file has a lot of usecases I wouldn't want to break.

My use case could be solved by doing something this.

entriesByFolder(collection, extension) {
    return this.api.listFiles(collection.get("folder"))
    .then(files => files.filter(file => file.name.endsWith(extension)))
    .then(this.fetchFiles);
  }

But all the other cases wouldn't work. Trailing slash, etc...

If the slug with language {{slug}}.{{language}} gets added as part of 2.0, this fix won't be necessary.

[tech4him1]

@Nic128 That's exactly the right fix, it was added to GitHub in #1123, but didn't get ported over to GitLab. Feel free to make a PR for that if you're up for it.