update 'accept' field

app.py

@@ -84,7 +84,7 @@ async def on_chat_start():
     while files == None:
         files = await cl.AskFileMessage(
             content="Please upload a Text File or PDF to begin!",
-            accept=["text/plain"],
+            accept=["text/plain", "application/pdf"],
             max_size_mb=2,
             timeout=180,
         ).send()