Replace `ext` opt arg to `filename` in Path() obj.

Path() datatype tmpfile behavior has changed.
Now, a random subdirectory is created inside
TMPPATH setting, then a file named `filename`
(which defaults to 'file.txt') is created inside him.
Change min php version from 4.1.0 to 4.3.0.

It means that the API, and plugin payloads are
now allowed to use php functions that do not existed
before php 4.3.0.
For example, file_get_contents() is now allowed.