added update to comments with characters counter

backend/app/routers/upload.py

@@ -0,0 +1,35 @@
+# app/routers/upload.py
+
+from fastapi import APIRouter, UploadFile, File, HTTPException
+import os
+
+router = APIRouter()
+
+
+@router.post("/upload")
+async def upload_file(file: UploadFile = File(...)):
+    if not file.filename.endswith('.xlsx'):
+        raise HTTPException(status_code=400, detail="Invalid file format. Please upload an .xlsx file.")
+
+    save_path = os.path.join("uploads", file.filename)
+    os.makedirs(os.path.dirname(save_path), exist_ok=True)
+    with open(save_path, "wb") as buffer:
+        buffer.write(await file.read())
+
+    # Validate the file (add your validation logic here)
+    is_valid, summary, error = validate_file(save_path)
+    if not is_valid:
+        raise HTTPException(status_code=400, detail=error)
+
+    return summary
+
+
+def validate_file(file_path: str):
+    # Implement your file validation logic here
+    # For demo purpose, assuming it always succeeds
+    summary = {
+        "dewars": 5,
+        "pucks": 10,
+        "samples": 100,
+    }
+    return True, summary, None