This is a workstation-specific issue. Please check the following:
  1. Is the attachment open somewhere else on the computer or by anyone else on the network?
  2. Is the file or the folder that contains the attachment password protected?
  3. Do you have a firewall or anti-virus program enabled that restricts sending certain file types?
Please address the above if any are applicable. If not:
  1. Advise copying the file and upload the new copy. 
  2. Advise submitting the application from a different computer. 
  3. If both fail, you may need to submit the application for the client with their log-in credentials or have the client email the attachment separately.