Within the IS document you presumably have a list of filenames. Loop over the list (refer to the docs for looping over a list that is within a document). For example, you might loop over \yourDoc\header\filenames with an output list of myFilenameList.
Then use the transformer within the loop to split each entry. You’d map from \yourDoc\header\filenames (which will appear to be a single string within the loop, rather than a list) to the transformer input. And map the transform output, say it outputs basename and extension, to fields within your myFilenameList var (which will appear to be a single document within the loop rather than a document list).
#Integration-Server-and-ESB#Flow-and-Java-services#webMethods