Client-side navigation for static sites fails if HTTP server serves .txt files (Next.js prefetch data) without Content-Type

### 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 <Link prefetch /> 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: https://github.com/vercel/next.js/blob/30a7204c3a4e0f0bbba18a9e5bcce0f4ca4738d9/packages/next/src/client/components/router-reducer/fetch-server-response.ts#L184-L203 ### Provide environment information ```bash 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/pla…