Add clearstatcache for fix a bug on specific configuration

[momo360modena]

This bug is present on the configuration of this ticket ticket:13659

With the WordPress importer, the method to import an attachment is :

• Create a empty file
• Get datas from the URL
• Write the datas on the empty file.
• Check size by compare Header and file size.

The bug is here.
When the filesize() is call, the function return 0 because the value is on cache from the empty file.

Call the clearstatcache() function solve the problem.

[ryan]

(In [15106]) clearstatcache() after writing files. Props momo360modena. fixes #13683