Why `insights-client --test-connection` showing 'HTTP Status Code: 400' for Upload URL Connection Test ?

Issue

• insights-client --test-connection fails with an error HTTP Status Code: 400 while doing === Begin Upload URL Connection Test === on Satellite 6 clients.

  # insights-client --test-connection 
  GET https://satellite6.redhat.com:443/redhat_access/r/insights/v1/branch_info
  GET branch_info status: 200
  Running Connection Tests...
  === Begin Upload URL Connection Test ===
  Testing: https://satellite6.redhat.com:443/redhat_access/r/insights/uploads/
  HTTP Status Code: 400   
  HTTP Status Text: Bad Request
  HTTP Response Text: File or upload field not found
  Connection failed
  === End Upload URL Connection Test: FAILURE ===
  
  === Begin API URL Connection Test ===
  Testing: https://satellite6.redhat.com:443/redhat_access/r/insights/
  HTTP Status Code: 200
  HTTP Status Text: OK
  HTTP Response Text: {}
  Successfully connected to: https://satellite6.redhat.com:443/redhat_access/r/insights/
  === End API URL Connection Test: SUCCESS ===
  
  Connectivity tests completed with some errors
  See /var/log/insights-client/insights-client.log for more details.