How can we get original file name of an uploaded file?

We can make use of the document() function to get the uploaded filename.

document(ri!docmentId,"name")

The original file name is inherently kept unless you override it, either at upload time or perhaps via renaming afterwards (in both cases, the original filename would no longer be stored anywhere within Appian as far as I know).  Otherwise, ashok's answer above is correct.  Do you mind sharing what you've tried and what seems to not be working correctly?