fix upload 500: add missing _file_suffix to serve server; fix iOS file picker accept types

bincio/serve/server.py

@@ -612,6 +612,14 @@ async def save_athlete(
 _SUPPORTED_SUFFIXES = {".fit", ".gpx", ".tcx", ".fit.gz", ".gpx.gz", ".tcx.gz"}
 
 
+def _file_suffix(name: str) -> str:
+    """Return the effective suffix, including .gz double-extension."""
+    p = Path(name.lower())
+    if p.suffix == ".gz":
+        return p.stem.rsplit(".", 1)[-1].join([".", ".gz"]) if "." in p.stem else ".gz"
+    return p.suffix
+
+
 @app.post("/api/upload")
 async def upload_activity(
     files: list[UploadFile] = File(...),