Altered AdminImportController::split() so that it uses in-memory (php://memory) file rather than generating a unique temp file on disk and writing a string to that, closing it, then re-opening it again in read mode just to be able to use fgetcsv() (instead of using the newer str_getcsv() function which isn't supported in PHP 5.2.0).  This fix should be much more efficient between only using memory and only opening and closing the in-memory file once.

This also fixes a bug that @vAugagneur fixed related to the filename getting lost and not being able to read it back properly. See PrestaShop/PrestaShop@8d7196daf3b39d280c631882f0e3ebe473233d07