The Broadcast cannot have a file uploaded to it because it is in a state other than PENDING_UPLOAD.
Possible Causes
The Broadcast SID that you provided is for a Broadcast that is in a state other than PENDING_UPLOAD. Those other states include: UPLOADED, QUEUED, EXECUTING, EXECUTION_FAILURE, EXECUTION_COMPLETED, CANCELATION_REQUESTED, CANCELED
Possible Solutions
Upload the file to a Broadcast SID that is in a PENDING_UPLOAD state