This is because uploading has been disabled in your PHP settings. To enable this you will need to modify your php.ini file to allow file uploading.
Make sure this line is visible and is not commented out:
file_uploads = On
Note that if there is a semi colon (;) at the beginning of that line, you will need to remove it.
If you do not have access to your php.ini file, then please contact your web hosting provider to enable file uploading.

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