Please note that Filesystem API is no longer part of the standard’s specification, as specified at:
http://www.w3.org/TR/file-system-api/
EDIT:
Quoting the specification in case the link changes:
“File API: Directories and System
W3C Working Group Note 24 April 2014
Work on this document has been discontinued and it should not be referenced or used as a basis for implementation.“
(This does not relate to the question directly, but it is essential to know not to use the FileSystem API further.)
Another link:
http://www.html5rocks.com/en/tutorials/file/filesystem/
“In April 2014, it was announced on public-webapps that the Filesystem API spec should be considered dead. Other browsers have showed little interest in implementing it.”