How does SQL bind rows in filestream-enabled table with their files
Just as caveat it is best not to disturb the physical files involved with FILESTREAM for a database.

Below are a few good links on the architecture:
Paul Randal White Paper - FILESTREAM Storage
Paul Randal Blog Post - FILESTREAM directory structure

As quoted from Paul's blog post above:


  The FILESTREAM file names are actually the log-sequence number from the database transaction log at the time the files were created. You can correlate these by looking at the data with DBCC PAGE, but first finding the allocated pages using sp_AllocationMetadata (see this blog post).


Note the stored procedure he mentions is an undocumented one that he provides some additional information to in the blog post he linked.