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<?php

/**
 * Tipo de arquivos aceitos (Lista em http://www.freeformatter.com/mime-types-list.html)
 * 
 * image/jpeg = jpg e jpge
 * image/gif = gif
 * image/png = png
 * application/pdf = pdf
 * application/vnd.openxmlformats-officedocument.wordprocessingml.document = docx
 * application/msword = doc
 */
$allowed = array("image/jpeg", "image/gif", "image/png");

if( in_array($_FILES["nomedoarquivo"]["type"], $allowed) ){
    echo 'Arquivo aceito!';
}