Feature: Add multipart/form-data support for file uploads

[fdarian]

Problem

Currently, tRPC procedures that accept file uploads via zod-form-data cannot be included in the generated OpenAPI specification. The library only supports application/json and application/x-www-form-urlencoded content types.

This forces endpoints like file upload to be disabled from the public API (enabled: false), limiting API capabilities.

Proposed Solution

Add automatic detection and proper schema generation for multipart/form-data:

1. Detect zod-form-data schemas - Identify when input uses zfd.formData() or contains zfd.file() fields.

2. Auto-set content-type - When file fields are detected, automatically use multipart/form-data instead of application/json.

3. Generate binary schema - File fields should generate:

   { "type": "string", "format": "binary" }

Example

// Schema using zod-form-data
const uploadSchema = zfd.formData({
  applicationId: z.string().optional(),
  zip: zfd.file(),
});

// tRPC procedure
uploadFile: protectedProcedure
  .meta({ openapi: { method: 'POST', path: '/upload' } })
  .input(uploadSchema)
  .mutation(...)

Generated OpenAPI:

/upload:
  post:
    requestBody:
      content:
        multipart/form-data:
          schema:
            type: object
            properties:
              applicationId:
                type: string
              zip:
                type: string
                format: binary
            required:
              - zip

Backward Compatibility

• Non-file schemas continue to use application/json
• No breaking changes to the API