I managed to loop through and extract the output code for each attachment from a start event form file upload control and end with just the file name. I which this was available through the collection of information storing the attachments, similar to SPO list item attachment collection.

Anyways, now how do I build an expression on the branch by condition action to check for special characters? I want to kick it back to the user if any exist to try again. So I will be setting a boolean variable to YES if special characters exist is the idea.