Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/modern-violet-m4ptx3?workspaceId=ws_WttkrM2Pr33FxG8UFgh8Lc

To Reproduce

1. Export a static Next.js site with output: 'export' set in next.config.js.
2. Host the output (out/) folder using a generic static HTTP server that serves .txt files without Content-Type.
3. Navigate to the site and inspect prefetch behavior for components pointing to static pages.
4. Observe that while the page should be rendered in place, clicking the link results in a full page reload.
5. In devtools, observe that the browser does fetch the correct .txt file (e.g., index.txt).

A live demo: https://next-prefetch-test.mountainaire.workers.dev/

Current vs. Expected behavior

Expected:
When prefetching a static page, Next.js should recognize the .txt response and perform client-side navigation if needed, as long as the content is correct and the file exists.

Observed:
If the HTTP server serves .txt files without header Content-Type: text/plain, Next.js fails this check and falls back to a full page navigation:

Provide environment information

Next.js config:

{
  output: 'export'
}

Which area(s) are affected? (Select all that apply)

Route Handlers

Which stage(s) are affected? (Select all that apply)

Other (Deployed)

Additional context

This issue happens when deploying a statically exported site to a somewhat strange static server that does not report the correct MIME type for .txt files in the response header. Because of how the Next.js router checks the Content-Type, even successful fetches of the .txt files are treated as invalid, resulting in broken prefetching and full page reloads.

Possible solution:

Loosening the check for valid prefetch responses to not rely on Content-Type: text/plain.