accept constraint for file inputs

[transparent-citizen]

File inputs can take an accept attribute to filter the filesystem dialog. It allows a comma-separated list of file type specifiers. These can either be a file extension (.png) or a MIME type (image/png).

As of zod v4, native file support is available through z.file. It doesn't seem to support file extensions, but there is support for MIME types through z.file().mime().

Given a z.file(), you can read the array of MIME types like this:

const fileSchema = z.file().mime(["image/jpeg", "image/png"]);

fileSchema.toJSONSchema().anyOf.map(x => x.contentMediaType) // ["image/jpeg", "image/png"]

It would be useful to have this data available in metadata for setting the accept attribute for a file input based on the schema.

[edmundhung]

Makes sense to me.

The current implementation doesn't rely on .toJSONSchema(). So we might need to evaluate what's the best approach to derive that. Maybe switching to it would simplify the setup too?

It will be fairly easy to hook this up once we figure out how to best derive it:

1. Add a new validation attribute: accept
2. Populate a new field metadata: accept

Interested in giving it a try?

[transparent-citizen]

I'm not sure if .toJSONSchema() would help simplify things on Conform's end. I was under the impression that interfacing against Standard Schema was supposed to be the way to unify validation libraries, but I'm not deeply familiar with either of them, and it looks like such implementation details are not part of the interface.

Internally, zod accesses the MIME types like this:

const fileSchema = z.file().mime(["image/jpeg", "image/png"]);

fileSchema._zod.bag.mime // ["image/jpeg", "image/png"]

It's kind of awkward to rely on accessing what seems to be internal zod state, but the MIME types don't seem to be exposed in any other way other than via .toJSONSchema(), which is more expensive to retrieve, but accessible through a public interface.

Do you prefer any way over the other? I can give it a try for sure.

[edmundhung]

Introspect a schema has always been challenging. Standard schema won't help as it only provide a simple validate interface and we have been relying on internal zod state for the implementation for a while.

It seems like .toJSONSchema is introduced only since v4.2.0. So I don't think we can depend on it.