This script can act as a middleman between Sentry's webhook system and Mattermost's Incoming Webhooks

sentry_mattermost_webhook.php

<?php
$json = json_decode(file_get_contents('php://input'), true);

if ($json['action'] !== 'created'){ die('Only create actions are alerted'); }
$error = $json['data']['issue'];

if ($error['project']['id'] == 1){ die('Ignore internal errors.'); }
if (empty($error['shortId'])){ die('No shortId passed'); }

// Set channel and username
$response['username'] = 'Sentry';
$response['response_type'] = 'in_channel';

$attachment = [];
$attachment['fallback'] = 'Error reported by Sentry: ' . $error['id'];
$attachment['author_name'] = strtoupper($error['project']['name']) . ' - ' . $error['shortId'];
$attachment['author_icon'] = 'ICON_HERE';
$attachment['author_link'] = 'PATH_TO_YOUR_ORG_HERE' . $error['id'];
$attachment['text'] = $error['title'] . ' @ ' . $error['culprit'];

$fields = [
        'Function' => $error['metadata']['function'],
        'File' => $error['metadata']['filename']
];

// Generate fields
foreach ($fields as $label => $value){
        $field = [
                'short' => true,
                'title' => $label,
                'value' => $value
        ];
        $attachment['fields'][] = $field;
}

$response['attachments'][] = $attachment;
$response = json_encode($response);

$ch = curl_init('YOUR_MATTERMOST_WEBHOOK');
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $response);
curl_setopt( $ch, CURLOPT_HTTPHEADER, array('Content-Type:application/json'));
echo curl_exec($ch);

For anybody that stumbled upon this and doesn't want to spin a PHP server, we implemented the same with a 2-step flow in N8N (same field mapping as the PHP which is quite elegant)

I've uploaded screenshots of the N8N config.

The result: