Looks like you are complicating this. You don't have to do any of that if all you trying to do is display a previously selected attachment. Please find attached a modified version where you can use display value field to select an attachment.
It is best to be done using the Java APIs associated with related contents. You can refer to an example at activiti-examples/activiti-copy-attachments-from-parent-process at master · cijujoseph/activiti-exam....
If it is one or two files, using the url field field is an option too. Please find attached an example showing how to do it. to run this example, you need to know the content id which you can obtain using rest apis.
Hope this helps. Also, when asking questions, it would be great if you can provide as much details as you can at the beginning including some attachments like unit tests, prototypes etc which will help save time!
Ciju, this still does not resolve my issue. The processes would not be related in any way; there is no parent-child relationship. I basically need to create a search engine where a user can find/download ANY content that has been uploaded in an entirely different process.
As I am unable to view your example, would you mind explaining what you have done? I understand I perform an API call to retrieve the file using its contentId. When I have done this, it returns the raw file in a way that does not let me map it to a variable which I can then display in another form. How do I get from the API call to being able to view/download the file?