Preseve file names in the file pool

This has been already reported in the forum:
https://moodle.org/mod/forum/discuss.php?d=321816

For sites working with files, renaming the files to a hash file name is problematic, we need to preserve the file name, I already discussed this with dpalou and we think that maybe we can preserve the original name and just append the hashname of the file at the end.

In this way, the file name will change but, at least, it will preserve the original file name.