We have a component workflow that is able to take a collection of files along with a regular expression, and use that regular expression to find a specific file in the collection. This is extremely helpful in situations where we need a file that we know the name of,but don't know which index in the collection the file is. The component workflow then puts the found file into its own collection and is available as an output variable to the workflow that called it.
This component workflow takes advantage of how collections/files are managed behind the scenes, but this isn't well documented. I'm fearful that this functionality may change at some point, breaking the component workflow that is heavily used in a number of workflows.
Please update Nintex Automation Cloud to natively support the ability to get a specific file from a collection, preferably with the ability to use a regular expression. Also, it would be very helpful if file variables could be passed from workflow to workflow without the need to wrap it in a collection.