File responses cannot set a Content-Type: the contentType option is ignored and HttpStaticServer serves every file as application/octet-stream #8252

What version of Effect is running?

4.0.0-rc.115 (also reproduced on 4.0.0-rc.112)

What steps can reproduce the bug?

A file response keeps a content type only when it is supplied at construction, through headers. Every other route fails silently, including HttpStaticServer's own mime table.

bun add effect@4.0.0-rc.115 @effect/platform-bun@4.0.0-rc.115
bun run repro.ts

import { Effect, Layer } from "effect"
import {
  Etag, HttpPlatform, HttpRouter, HttpServerResponse, HttpStaticServer,
} from "effect/unstable/http"
import { BunServices } from "@effect/platform-bun"
import { mkdtempSync, writeFileSync } from "node:fs"
import { tmpdir } from "node:os"
import { join } from "node:path"

const root = mkdtempSync(join(tmpdir(), "static-"))
const file = join(root, "app.js")
writeFileSync(file, "console.log(1)")

const show = async (label: string, route: Layer.Layer<never, never, never>) => {
  const { handler, dispose } = HttpRouter.toWebHandler(
    route.pipe(
      Layer.provideMerge(HttpPlatform.layer),
      Layer.provideMerge(Etag.layerWeak),
      Layer.provideMerge(BunServices.layer),
    ) as never,
    { disableLogger: true },
  )
  const res = await handler(new Request("http://localhost/app.js"))
  console.log(label.padEnd(46), "->", res.headers.get("content-type"))
  await dispose()
}

await show("A. file(path, { contentType })",
  HttpRouter.add("GET", "/*", HttpServerResponse.file(file, { contentType: "text/javascript" }) as never))

await show("B. file(path, { headers: content-type })",
  HttpRouter.add("GET", "/*", HttpServerResponse.file(file, { headers: { "content-type": "text/javascript" } }) as never))

await show("C. HttpStaticServer.make({ root })",
  Layer.unwrap(Effect.map(HttpStaticServer.make({ root }), (files) => HttpRouter.add("GET", "/*", files as never))))

await show("D. file(path) then setHeaders Content-Type",
  HttpRouter.add("GET", "/*", Effect.map(HttpServerResponse.file(file), HttpServerResponse.setHeaders({ "Content-Type": "text/javascript" })) as never))

await show("E. file(path) then setHeaders content-type",
  HttpRouter.add("GET", "/*", Effect.map(HttpServerResponse.file(file), HttpServerResponse.setHeaders({ "content-type": "text/javascript" })) as never))

On Node, swap BunServices from @effect/platform-bun for NodeServices from @effect/platform-node; the code under test is platform independent.

What is the expected behavior?

A. file(path, { contentType })                 -> text/javascript
B. file(path, { headers: content-type })       -> text/javascript
C. HttpStaticServer.make({ root })             -> text/javascript; charset=utf-8
D. file(path) then setHeaders Content-Type     -> text/javascript
E. file(path) then setHeaders content-type     -> text/javascript

C should come from the built-in table, which already maps js to text/javascript; charset=utf-8.

What do you see instead?

A. file(path, { contentType })                 -> application/octet-stream
B. file(path, { headers: content-type })       -> text/javascript
C. HttpStaticServer.make({ root })             -> application/octet-stream
D. file(path) then setHeaders Content-Type     -> application/octet-stream
E. file(path) then setHeaders content-type     -> application/octet-stream

Only B is correct. A accepts an option that is never read; D and E show the header does not stick after construction, in either casing.

Additional information

The practical consequence is that HttpStaticServer cannot serve a JavaScript bundle or a stylesheet: a browser refuses the script with a MIME type error, and index.html is offered as a download rather than rendered.

Observations, offered without claiming a diagnosis:

• HttpServerResponse.file's options type includes contentType, but HttpPlatform.fileResponse reads only status, statusText, headers, offset and bytesToRead, so nothing consumes it.
• HttpStaticServer.setFileHeaders sets Content-Type and Accept-Ranges on an already-built response. Accept-Ranges arrives; Content-Type does not. So the mutation is not lost wholesale, only for this header.
• setHeaders calls makeResponse(..., true), and the preferHeaders branch in HttpServerResponse.ts looks like it should keep a caller's content-type for a file body, which it does not.

Possibly adjacent to #8146 and #8148, which concerned Content-Type being dropped from file responses during compression. This happens with no compression involved.

Workaround, for anyone who hits it: pass the type through headers at construction and compute it yourself. That means bypassing HttpStaticServer, since its own table cannot be made to apply.