Add check to see if a file has already been uploaded

[Tiago Pereira]

Is it possible to implement an check on a transition to see if a file has already been uploaded on attachment?

[Frederik Vantroys]

You can 

Add a "Field Required" validator to the transition, targeting the "Attachments" field. This validator ensures that the "Attachments" field is not empty before the transition can proceed.

But if you really want more advanced checks you will need Plugins like ScriptRunner, Automation for Jira, or Jira Workflow Toolbox offer additional customization options, including more complex attachment validation rules.