feat(server): add content-type map for preview-fs static route

Implement a pure function that maps file extensions to their correct MIME types,
supporting common web assets (HTML, CSS, JS, JSON, images, fonts) with proper
charset declarations and fallback to application/octet-stream.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/server/api/__tests__/previewFs.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from 'bun:test'
+import { contentTypeForPath } from '../previewFs'
+
+describe('contentTypeForPath', () => {
+  it('maps common web asset extensions', () => {
+    expect(contentTypeForPath('/x/index.html')).toBe('text/html; charset=utf-8')
+    expect(contentTypeForPath('/x/app.css')).toBe('text/css; charset=utf-8')
+    expect(contentTypeForPath('/x/app.js')).toBe('text/javascript; charset=utf-8')
+    expect(contentTypeForPath('/x/data.json')).toBe('application/json; charset=utf-8')
+    expect(contentTypeForPath('/x/logo.svg')).toBe('image/svg+xml')
+    expect(contentTypeForPath('/x/p.png')).toBe('image/png')
+  })
+  it('falls back to octet-stream for unknown', () => {
+    expect(contentTypeForPath('/x/file.bin')).toBe('application/octet-stream')
+  })
+})

src/server/api/previewFs.ts

@@ -0,0 +1,26 @@
+import * as path from 'node:path'
+
+const CONTENT_TYPES: Record<string, string> = {
+  html: 'text/html; charset=utf-8',
+  htm: 'text/html; charset=utf-8',
+  css: 'text/css; charset=utf-8',
+  js: 'text/javascript; charset=utf-8',
+  mjs: 'text/javascript; charset=utf-8',
+  json: 'application/json; charset=utf-8',
+  svg: 'image/svg+xml',
+  png: 'image/png',
+  jpg: 'image/jpeg',
+  jpeg: 'image/jpeg',
+  gif: 'image/gif',
+  webp: 'image/webp',
+  ico: 'image/x-icon',
+  woff: 'font/woff',
+  woff2: 'font/woff2',
+  txt: 'text/plain; charset=utf-8',
+  md: 'text/plain; charset=utf-8',
+}
+
+export function contentTypeForPath(filePath: string): string {
+  const ext = path.extname(filePath).slice(1).toLowerCase()
+  return CONTENT_TYPES[ext] ?? 'application/octet-stream'
+}