$fileAttachment = null;

$_POST['id_contact'] = 3;

<p class="select">
                <label for="id_contact">{l s='Subject Heading'}</label>
            {if isset($customerThread.id_contact)}
                {foreach from=$contacts item=contact}
                    {if $contact.id_contact == $customerThread.id_contact}
                        <input type="text" id="contact_name" name="contact_name" value="{$contact.name|escape:'htmlall':'UTF-8'}" readonly="readonly" />
                        <input type="hidden" name="id_contact" value="{$contact.id_contact}" />
                    {/if}
                {/foreach}
            </p>
            {else}
                <select id="id_contact" name="id_contact" onchange="showElemFromSelect('id_contact', 'desc_contact')">
                    <option value="0">{l s='-- Choose --'}</option>
                {foreach from=$contacts item=contact}
                    <option value="{$contact.id_contact|intval}" {if isset($smarty.post.id_contact) && $smarty.post.id_contact == $contact.id_contact}selected="selected"{/if}>{$contact.name|escape:'htmlall':'UTF-8'}</option>
                {/foreach}
                </select>
            </p>
            <p id="desc_contact0" class="desc_contact">&nbsp;</p>
                {foreach from=$contacts item=contact}
                    <p id="desc_contact{$contact.id_contact|intval}" class="desc_contact" style="display:none;">
                        <label>&nbsp;</label>{$contact.description|escape:'htmlall':'UTF-8'}
                    </p>
                {/foreach}
            {/if}