If you're using upload-by-url you need to have
$wgAllowCopyUploads=true and you're user needs to have the proper
userright (can't remember which one, look it up in the docs). You
don't need multipart/form-data for upload-by-url, only for regular
file uploads. Also, for regular file uploads, your file field needs to
be of the file-type, that is it requires the filename attributes and
perhaps some others.

I have this in my LocalSettings:
$wgEnableUploads = true;
$wgFileExtensions = array('png', 'gif', 'jpg', 'jpeg');
$wgAllowCopyUploads = true;
$wgGroupPermissions['user']['upload_by_url'] = true;
$wgGroupPermissions['user']['upload'] = true;

I have proved to upload from url with and without multipart/form-data and also from file. The problems persists. I just wanna create a simple php code to upload to my wiki, how can this be such an annoying task? :-(