Detect text files via MIME + binary heuristic instead of hardcoded extension list

Server-side _file_type() now uses mimetypes for images/known text,
and falls back to null-byte check on first 8KB for everything else.
Client uses the "type" field from /files API instead of local extension sets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

share.py

@@ -74,6 +74,18 @@ def ensure_cert(cert_dir: Path, extra_ips: list[str] | None = None) -> tuple[Pat
 
 BASE_DIR = Path(__file__).resolve().parent
 
+def _file_type(path: Path) -> str:
+    ct = mimetypes.guess_type(path.name)[0] or ''
+    if ct.startswith('image/'):
+        return 'image'
+    if ct.startswith('text/'):
+        return 'text'
+    try:
+        with open(path, 'rb') as f:
+            return 'text' if b'\x00' not in f.read(8192) else 'other'
+    except OSError:
+        return 'other'
+
 
 # ── HTTP Handler ──
 
@@ -149,7 +161,7 @@ class Handler(http.server.BaseHTTPRequestHandler):
         files = []
         for p in sorted(self.server.upload_dir.iterdir(), key=lambda p: p.stat().st_mtime, reverse=True):
             if p.is_file():
-                files.append({"name": p.name, "size": p.stat().st_size})
+                files.append({"name": p.name, "size": p.stat().st_size, "type": _file_type(p)})
         self._json(files)
 
     def _route_download(self):