allow PDF file type

app.py

@@ -86,7 +86,7 @@ async def on_chat_start():
     while files == None:
         files = await cl.AskFileMessage(
             content="Please upload a PDF File file to begin!",
-            accept=["text/plain"],
+            accept=["application/pdf"],
             max_size_mb=2,
             timeout=180,
         ).send()