All you need to do is open LocalSettings.php
and add or change this this lines
To enable image uploads, make sure the 'images' directory
is writable, then set this to true:
$wgEnableUploads = false;
change the false for true and you're done...
Also if you need to upload more things than images only you need to define what files do you allow.
For example I use:
$wgFileExtensions = array('png', 'gif', 'jpg', 'jpeg', 'doc', 'xls', 'pdf');
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