irule for persistent

When does qarpc change? You need to study the logging of the qarpc cookie and see when it changes as a file transfer occurs. The F5 will always persist back to the same pool member if the cookie matches.

Logging is the key to solving your problem. You need to watch the cookie behaviour throughout a session. This will help you.

when CLIENT_ACCEPTED {
  set cid [expr {int (rand() * 100000)}]
  log local0. "\[$cid\] Connection opened."
}
when CLIENT_CLOSED {
  log local0. "\[$cid\] Connection closed."
}
when HTTP_REQUEST {
  set rid [expr {int (rand() * 100000)}]
  log local0. "\[$cid:$rid\] URI = [string range [HTTP::uri] 0 50]..."
  if {[HTTP::cookie exists qarpc]} {
    log local0. "\[$cid:$rid\] Cookie qarpc = [string range [HTTP::cookie qarpc] 0 50]..."
  } else {
    log local0. "\[$cid:$rid\] No Cookie qarpc Found In Request"
  }
}
when HTTP_RESPONSE {
  if {[HTTP::cookie exists qarpc]} {
    log local0. "\[$cid:$rid\] Response Cookie qarpc = [string range [HTTP::cookie qarpc] 0 50]..."
  }
}