When you upload attachments to a post/page they become child elements of that post/page.
What you can do is provide a drop down in your custom field to list all uploaded files to the uploader for the current post/page. This is the code I used to show a list of files so the user could select which one was a floor plan image:
<select name="chb_homes_for_sale_specifics_floor_plan" style="width:100%;">
<option value="" <?php selected($custom["chb_homes_for_sale_specifics_floor_plan"], ""); ?>>No Floor Plan</option>
$args = array(
'numberposts' => -1,
'orderby' => 'menu_order',
'order' => 'ASC',
'post_type' => 'attachment',
'post_parent' => $post->ID,
'post_mime_type' => 'image'
$image = get_posts($args);
foreach($image as $key => $data) : ?>
<option value="<?php echo $data->ID; ?>" <?php selected($custom["chb_homes_for_sale_specifics_floor_plan"], $data->ID); ?>><?php echo basename ( get_attached_file( $data->ID ) ); ?></option>