Giving into attention code tag (#) you can use for inserting code here.
This is part I was talking about:
<?php if(osc_item_attachment()) { echo 'enctype="multipart/form-data"'; };?>

I didn't understand your last question, do I need to do something with the code you wrote?

(I did a test by putting the code here:

item-send-friend.php
..............................................
<!-- ITEM CONTACT FORM -->
    <form target="_top" action="<?php echo osc_base_url(true) ; ?>" method="post" name="contact_form" id="contact_form" <?php if(osc_item_attachment()) { echo 'enctype="multipart/form-data"'; };?> class="fw-box" style="display:block;">
      <input type="hidden" name="action" value="contact_post" />
      <input type="hidden" name="page" value="item" />
      <input type="hidden" name="id" value="<?php echo osc_item_id() ; ?>" />

but it didn't work)

yes it's needed as form must have enctype="multipart/form-data" attribute to be able to transfer attachments.

It worked!!! I placed the code as you said and disabled the "Form submission without reloading" (Ajax) which was enabled until now and everything works fine. Thanks for your help and patience.

Welcome.
Files cannot be submitted via ajax.