Manual:$wgUploadDirectory

Details
The file system path of the folder where uploaded files will be stored.

If this variable is changed, you may need to update  accordingly.

This location needs to be writable by the web server. See 1>Special:MyLanguage/Manual:Configuring file uploads#Check directory security|Manual:Configuring file uploads#Check directory security.

By default, uploaded files are actually stored in subfolders of $wgUploadDirectory. See 1>Special:MyLanguage/Manual:Configuring file uploads#Upload directory|Manual:Configuring file uploads#Upload directory for information, and how to disable this behavior.