Adding a .mp4 asset seems to silently fail.

I'm want to make a .mp4 file available to download. I have a page with the following template

<?php
    $pageTitle = perch_pages_title(true);
    $pageType = 'default with-sidebar';
    include($_SERVER['DOCUMENT_ROOT'].'/includes/header.php');

    perch_content_create('Main Content', array(
        'template' => 'main_content.html',
    ));

    perch_content_create('Sidebar', array(
        'template' => 'blocks.html',
    ));
?>    

<section>
    <div class="container">
        <div class="row">
            <div class="col-md-8">

                <?php perch_content('Main Content'); ?>

            </div>

            <div class="col-md-3">

                <?php perch_content('Sidebar'); ?>

            </div>
        </div>
    </div>
</section>

<?php include($_SERVER['DOCUMENT_ROOT'].'/includes/footer.php'); ?>    

I've selected "File" type for "Main Content".

Then I choose select/upload a file and the asset manger pop up appears.

Then I click "Add asset."

I've tried drag dropping the .mp4 in and I've clicked and then browsed for the .mp4.

Both ways it shows "Uploading test.mp4 (0.3 GB)" (actually called "test.mp4")

The message stays for a while and then disappears.

There's no sign of the file anywhere in the asset manager and I've used filezilla ftp client to search my entire file system and it finds nothing.

Here is the Diagnostic Report:

Perch Runway: 3.0.13, PHP: 7.1.13, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: b396954eeb2d1d9ed7902b8bae237b287f21ad9e $, with PDO
Server OS: Linux, fpm-fcgi
Installed apps: content (3.0.13), assets (3.0.13), categories (3.0.13), perch_forms (1.10), perch_members (1.6.1)
App runtimes: <?php $apps_list = [ 'perch_forms', 'perch_members', ];
PERCH_LOGINPATH: /perch
PERCH_PATH: /var/www/vhosts/lighthousecentral.org/perch.lighthousecentral.org/perch
PERCH_CORE: /var/www/vhosts/lighthousecentral.org/perch.lighthousecentral.org/perch/core
PERCH_RESFILEPATH: /var/www/vhosts/lighthousecentral.org/perch.lighthousecentral.org/perch/resources
Image manipulation: GD Imagick
PHP limits: Max upload 16M, Max POST 16M, Memory: 128M, Total max file upload: 16M
F1: 3b606135b33e6a102526838f4152a807
Resource folder writeable: Yes
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php
REQUEST_URI: /perch/core/settings/diagnostics/
DOCUMENT_ROOT: /var/www/vhosts/lighthousecentral.org/perch.lighthousecentral.org
HTTP_HOST: perch.lighthousecentral.org

Thanks in advance

Sam