diff --git a/src/upload_handler.py b/src/upload_handler.py
index 61df395..8c8b2bd 100644
--- a/src/upload_handler.py
+++ b/src/upload_handler.py
@@ -43,6 +43,20 @@ def is_valid_upload_id(upload_id: str) -> bool:
     return UPLOAD_ID_RE.fullmatch(upload_id or "") is not None
 
 
+def _build_upload_id(safe_filename: str) -> str:
+    """Build a unique upload id whose extension matches UPLOAD_ID_RE.
+
+    secure_filename keeps '_' and '-', so an extension like '.jpg-1' (the
+    suffix browsers append to duplicate downloads) or '.v1_final' produced an
+    id that failed is_valid_upload_id, making the saved file permanently
+    unreadable (every read path gates on validate_upload_id). Sanitize the
+    extension to the single-alnum shape the id contract requires.
+    """
+    _, ext = os.path.splitext(safe_filename or "")
+    ext = re.sub(r"[^A-Za-z0-9]", "", ext)
+    return uuid.uuid4().hex + (("." + ext) if ext else "")
+
+
 def count_recent_uploads(timestamps, now: float, window: float = 10.0) -> int:
     """Number of upload events in *timestamps* within the last *window* seconds.
 
@@ -596,8 +610,7 @@ class UploadHandler:
                 }
         
         # Generate unique ID and determine save location
-        _, ext = os.path.splitext(safe_filename)
-        file_id = f"{uuid.uuid4().hex}{ext}"
+        file_id = _build_upload_id(safe_filename)
         
         # Create date-based directory structure
         upload_dir = self.get_upload_dir()
diff --git a/tests/test_upload_id_extension.py b/tests/test_upload_id_extension.py
new file mode 100644
index 0000000..70e2613
--- /dev/null
+++ b/tests/test_upload_id_extension.py
@@ -0,0 +1,37 @@
+"""Upload ids must satisfy UPLOAD_ID_RE for every accepted filename.
+
+secure_filename keeps '_' and '-', so a filename whose final extension
+contains them (e.g. "photo.jpg-1" — the suffix browsers add to duplicate
+downloads, or "doc.v1_final") produced an id like "<hex>.jpg-1" that fails
+is_valid_upload_id. Since every read path (download, resolve, vision)
+validates the id first, the saved bytes became permanently unreachable.
+"""
+import pytest
+
+from src.upload_handler import _build_upload_id, is_valid_upload_id
+
+
+@pytest.mark.parametrize("name", [
+    "photo.jpg-1",
+    "doc.v1_final",
+    "invoice.2024-01",
+    "file.JPG_backup",
+    "report.pdf",
+    "image.png",
+    "noextension",
+    "",
+])
+def test_built_id_is_always_valid(name):
+    fid = _build_upload_id(name)
+    assert is_valid_upload_id(fid), (name, fid)
+
+
+def test_normal_extension_is_preserved():
+    assert _build_upload_id("photo.png").endswith(".png")
+    assert _build_upload_id("doc.pdf").endswith(".pdf")
+
+
+def test_problem_extension_is_sanitized_not_dropped_to_invalid():
+    fid = _build_upload_id("photo.jpg-1")
+    assert is_valid_upload_id(fid)
+    assert fid.endswith(".jpg1")  # the '-' is stripped, alnum kept