correct pdf type

app.py

@@ -87,10 +87,10 @@ async def on_chat_start():
     files = None
 
     # Wait for the user to upload a file
-    while files == None:
+    while not files:
         files = await cl.AskFileMessage(
             content="Please upload a .txt or .pdf file to begin!",
-            accept=["text/plain", "pdf"],
+            accept=["text/plain", "application/pdf"],
             max_size_mb=2,
             timeout=180,
         ).send()