Found a fix! Changed WP_Query to query_posts() if someone else faces the same problem...
$args = array(
'post_type' => 'attachment',
'post_status' => 'inherit',
'posts_per_page' => -1,
'post_mime_type' =>'application/pdf, image/jpeg, image/gif, image/jpg, image/png'
);
$tmp = $post;
// The Query
$the_query2 = query_posts( $args );
if ( $the_query2 ) {
echo '<select class="bw_selector" name="bw_vendor_material">';
echo '<option value="">Choose attachment:</option>';
// The Loop
foreach ($the_query2 as $key => $value) {
//print_r($value->ID);
echo '<option value="'.$value->ID.'">' . $value->post_mime_type .': '. get_the_title($value->ID) . '</option>';
}
echo '</select>';
echo '<br/>';
}
else {
echo "No posts found";
}
// Restore original Post Data
wp_reset_query();
$post = $tmp;