On the listing page you see the green button to send a reaction to the listing poster.
Here you can add a phone number and the message itself (if you are logged in) otherwise you have to fill in your name and email.

Would it also be possible to upload a picture to it and send this picture with the reaction to it as a attachment?

@vansebas
You can enable attachments in contact form in listing settings.

I already enabled that option and in the contact form on the contact page I see the attachement field.
Only when I open a listing and click on the green to send a e-mail to the person the send a message field show but there is no attachment field (see attached image), this happend with the Gamma theme.

In item-send-friend.php find this line:
            <div class="row">
              <label for="message"><span><?php _e('Message', 'gamma'); ?></span><span class="req">*</span></label>
              <?php ContactForm::your_message(); ?>
            </div>

Bellow it add this code:
            <?php if(osc_item_attachment()) { ?>
              <div class="row">
                <label for="attachment"><?php _e('Attachment', 'gamma'); ?>:</label>
                <div class="input-box"><?php ContactForm::your_attachment(); ?></div>
              </div>
            <?php } ?>

Form submit without reload (Ajax) - should be disabled, as file cannot be uploaded and sent via ajax.

With that code I now see indeed the attachment field, only when I send the email I receive the email in my mailbox but without any attachment.

And AJAX send form option is disabled in the settings.

Will test it.

Will test it.

Any update on this?

I solved it!

I replaced this code of the adding attachment field with the same code from the contact page (so the looks are the same)
            <?php if(osc_item_attachment()) { ?>
              <div class="row">
                <label for="attachment"><?php _e('Attachment', 'gamma'); ?>:</label>
                <div class="input-box"><?php ContactForm::your_attachment(); ?></div>
              </div>
            <?php } ?>

And to the form tag  I added a code.
Default form tag is:
<form target="_top" action="<?php echo osc_base_url(true) ; ?>" method="post" name="contact_form" id="contact_form" class="fw-box" style="display:block;">
I changed it to:
<form target="_top" action="<?php echo osc_base_url(true) ; ?>" method="post" name="contact_form" <?php if(osc_contact_attachment()) { ?>enctype="multipart/form-data"<?php } ?> id="contact_form" class="fw-box" style="display:block;">
And now when I send a reaction the attachment is added to the email!

Good work