After digging, i have found that the problem was in the function packages/kcfinder/core/class/uploader.php :: checkFilePath
This function forbids an upload if you have a symbolic link in your path.
There is also a related issue in CiviCRM JIRA : https://issues.civicrm.org/jira/browse/CRM-15607
EDIT:
Probably not a very good solution but you can patch kcfinder.
In my case it's complicated to remove the symbolic link so I used this as a temporary fix.
Add the following line as the first line of the function checkFilePath to avoid the verification:
protected function checkFilePath($file) {
+ return True;
$rPath = realpath($file);
if (strtoupper(substr(PHP_OS, 0, 3)) == "WIN")
$rPath = str_replace("\\", "/", $rPath);
return (substr($rPath, 0, strlen($this->typeDir)) === $this->typeDir);
}