What is FID and how does it works?
You can find fid under the table name file_managed.
It is unique id for files. In files you can include images, .txt, .pdf etc extension files.
This fid is unique for the site and auto incremented
By using file_load($fid) function. Where $fid is file id.


Fid is only valid for managed files. Files uploaded as unmanaged does not have records in database, so no fid.