Sounds like you should be using a RouteOnAttribute processor between your listFile and FetchFile processor so that only those FlowFIles you want to Fetch the content for are routed to the FetchFile processor.
The listFile processor produces one FlowFile for each file that is returned during the listing execution. The listFile processor writes a number of FlowFile attributes to the FlowFiles that are produced.
The Filename attribute is what you will want to use when writing your NiFi Expression Language (EL) statement for your dynamically created properties added to the RouteOnAttribute processor.
You can find a EL guide under "Help" under the NiFi global menu in the upper right corner of the NiFi UI.