How are you specifying the filename right now in the File Reader Snap? If you make the filename a pipeline parameter, you’ll have access to it throughout the pipeline:
Then you can use string functions in a mapper to create the name you want. (It may be easier if you make the path and the file separate vars, and then do a _path + _file in the reader. Note that pipeline parameters are referenced with a leading _, and not a $.)
These are the string functions - if you can give a bit more information about the pipeline I can be more specific. http://doc.snaplogic.com/strings
Thanks,
Shayne
