It does not cover to a 100% what you are looking for, but Get (DocumentsPathListing) at least informs about the content of this folder. A "Get Directory Path Listing [ <directory> ]" step would be a helpful extension.
In short, there is no script step to list directory ( with exception of Get (DocumentsPathListing), already mentioned by Torsten Bernhard) .
You can use plugins. Troi file comes to mind, but there is a number of free plugins doing this as well.
If you need to stay free of plugins, a simple AppleScript for Mac OSX or VBScript (not so simple) or a batch script will do the job. With VBScript you will need to figure out how to return the list back to FileMaker.