Netlify-cms: Break on unsupported file format.

[tech4him1]

Currently, if an unsupported file format is set on a collection (format: ini), we simply assume that it should be saved as YAML front-matter (see #748 and #776). I think in this case, when the user has explicitly set a format, and we are not inferring at all, it makes more sense to just throw an "Invalid Config" error. @erquhart @Benaiah?

Expanded discussion: https://github.com/netlify/netlify-cms/issues/776#issuecomment-347731448

[tech4him1]

We could throw on extension as well, but there may be a valid use case where someone needs to have a different extension than the normal one for a filetype. It does make it hader for the user to know that we are inferring, though.

[Benaiah]

I think throwing on an unsupported format is a good idea. I think we should also throw if the extension is unsupported and there _isn't_ a valid format set. We shouldn't error if there is an unknown extension but a valid format.

[tech4him1]

Good point, then if people want to force an abnormal extension, they have to explicitly set the format.

[tech4him1]

Current status (after #796):

| format | extension | File | Formatter | |
|----------|-------------|-------|---------|-|
| | | md | md |
| known | | known | known |
| unknown| | md | md | Should throw.
| | known | known | known |
| | unknown | undefined | md | Broken + Should throw.
| unknown| known | md | md | Should throw.
| unknown| unknown | unknown | md | Should throw.

[tech4him1]

@Benaiah @erquhart Or should we not even try to infer from the extension at all -- just throw if a format is not set along with the extension?

[erquhart]

@tech4him1 nice table! This conversation is happening in a few places, is there a canonical issue or PR that we can condense down to?

[tech4him1]

Sure, most of the discussion here relates to https://github.com/netlify/netlify-cms/issues/776, I'll move it there.

[tech4him1]

The core of this issue is still just throwing on an unsupported format, so I'll leave it open until that gets merged. I moved the other discussion to #776.

[tech4him1]

@Benaiah @erquhart Should we account for "common misspellings" on the formats? yml vs yaml, md vs markdown? Right now it's really weird -- we account for the misspellings, but the extension is saved as undefined. We either need to throw on misspellings and provide a list of supported options, or just accept them as normal options.

[Benaiah]

@tech4him1 Yes, I think those alternative format names should be aliases for the ones we use.

[erquhart]

We just can't change the extension on existing entries (don't want to bother with the tree manipulation to produce a rename). But yeah, that should work.

[tech4him1]

@erquhart What about the format: frontmatter option? https://github.com/netlify/netlify-cms/blob/2ecc5355d9fe55c84e78f5f315d6b822dfbfa0de/src/formats/formats.js#L43 Is there a reason that needs be be in there, instead of using format: markdown, since right now the default file extension for it is undefined?

[erquhart]

If we don't need it, let's drop it.