Add a list of supported document types for source-files

[seancdavis]

Double check, but I believe it's:

• Markdown (with various extensions)
• MDX
• JSON
• YAML

Might be worthwhile to note what it is that tells Contentlayer how to process content. Is it the file extension?

cc @schickling

[schickling]

Might be worthwhile to note what it is that tells Contentlayer how to process content. Is it the file extension?

It's not the file extension but the contentType.

[seancdavis]

@schickling And if contentType is not set, does it fall back to the file extension, or does it have a static default (e.g. markdown?)

[schickling]

It always defaults to markdown right now. While it might sound intuitive to derive the content type based on the file extension of a given file, it's not what you want in most cases, as you need a "static guarantee" that all documents adhere to the same content type.

What we could consider doing though is derive the content type dynamically as long as all pieces of content have the same file extension. If that's not the case, show an error and ask the user to either specify a contentType or change the file extension. (I kind of like this idea.)