Might I ask you what can be the cause and solution of:
the automatic dissapearing of "large" files (±50MB, such as an audio file) that I have manually put into the root folder of my Drupal website/installation, using a FTP-client?
Does this has something to do with CRON (just a wild guess)? Is it not smart to manually (FTP) upload files into the root folder? Should a module be used at all times?
admin/config/media/file-system
sites/default/files
by default, which is the same folder as user skh was referring to in the answer below.