Hi @matself, the PATH reader just reads the information of files such as file paths, file names, extension etc. If you need to read the raster files after filtering their file paths by the TestFilter, consider using the FeatureReader transformer.
                
     
                                    
            Thanks @takashi. That pointed me in the right direction.
So here is what I did. I filtered out the three files 
and piped them through the initiator of the FeatureReader as suggested. 
But I was not able to figure out how to create three outport ports 
dynamically, by using the filename or any other attribute. So I ended up
 by taking the Generic port and TestFilter that output to split the flow
 into three streams. Basically the same test as before the FeatureReader
 which seems like an awful convoluted detour. Anyway, from that point on
 the workspace ran like before, and I can now process the RGB bands from
 a standard Sentinel scene into a natural color image only by selecting 
the folder path.
                
     
                                    
            Thanks @takashi. That pointed me in the right direction.
So here is what I did. I filtered out the three files 
and piped them through the initiator of the FeatureReader as suggested. 
But I was not able to figure out how to create three outport ports 
dynamically, by using the filename or any other attribute. So I ended up
 by taking the Generic port and TestFilter that output to split the flow
 into three streams. Basically the same test as before the FeatureReader
 which seems like an awful convoluted detour. Anyway, from that point on
 the workspace ran like before, and I can now process the RGB bands from
 a standard Sentinel scene into a natural color image only by selecting 
the folder path.
Firstly you can filter the file paths with the Path Filter parameter in the PATH reader. 
See here to learn more about the Path Filter: Directory and File Pathnames Reader Parameters 
Then, you can filter the raster features based on their file names if you exposed "fme_basename" which stores source file name without extension, in the FeatureReader.
 
e.g.
 
