producer can't send byte string of an image file

I tried to send sync a byte string read from an image to RMQ by the following code:
```
with open("model/test.jpeg", "rb") as f:
img_str = f.read()
msg.set_body(img_str)
ret = producer.send_sync(msg)
```
but the message body a consumer received is only a part of the byte string
```
b'\xff\xd8\xff\xdb'
```

Start by reproducing the issue with the producer snippet using set_body and send_sync, then trace how the message body is handled before the consumer receives it. Done means the consumer receives the complete image byte string rather than only its initial bytes; add or update a regression test if the repository has coverage for this path.

Written by the indexing model from the issue text.