docs: clarify file upload openapi

[nightspite]

I have lost some time looking for a way to extend the schema to handle file upload properly - correct openapi representation was worth more for me than the typed input. In swagger this field is displayed as a file input

https://swagger.io/docs/specification/describing-request-body/file-upload/

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[Gabrola]

Unfortunately, this would break the typing for both the client and server, and also fail validation on the server. Maybe this works for you in a specific use-case, but we cannot include this in the official docs.

[nightspite]

How will that fail a validation on the server? Current version in the docs specifies type only, no validation there.

contentType: 'multipart/form-data', // <- Only difference
body: c.type<{ thumbnail: File }>(), // <- Use File type in here

And as far as I remember the multipart content type is not respected in openapi schema.