If you get the following message:

Sorry, uploading is disabled.

you have to open MediaWiki configuration file (LocalSettings.php) at your web hosting account and do the following changes:

MediaWiki 1.5 or above:

$wgEnableUploads = true; # Enable uploads

Older versions:

Invert the value to enable uploads:

$wgDisableUploads = false; # Enable uploads

If you have problems enabling the MediaWiki file uploads yourself, ask your host for assistance. SiteGround provides the best MediaWiki hosting and such requests are handled with ease.