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The new option looks good to me but can you please elaborate more about "to work with custom extension files" usecase?

Here is an example, in UnoCSS's docs, we'd like to use markdown files as Vue components so we installed vite-plugin-md to transform md to Vue SFC and let Vue's compiler handle the rest. We could configure Vue by this so it treats .md as Vue SFC:

https://github.com/antfu/unocss/blob/9a2bed77e8a3433454aab66f9c3ea2b2bbb7bcf6/interactive/nuxt.config.ts#L33-L35

https://github.com/antfu/unocss/blob/6aa5f9fad9dda65ffceef1455566b4233f7db278/interactive/modules/markdown.ts

With this we could get it work, but the .md will not be processed by auto imports thus you lost the capability of using components in the markdown. This PR simply expose those options for that capability.

Thanks for explaining @antfu. What do you think to integrate include option to auto-include options.extensions

For API auto import I think it might make sense, but probably not for Components right?

Or should we have options.vueExtensions that we also pass to Vue's compiler?

probably not for Components right?

It could be also useful for building components with markdown (https://mdxjs.com/ https://mdx.nuxtjs.org/)