Added safe mime sniffing to prevent serving HTML

(Amoung other content types)
For #3027

app/Util/WebSafeMimeSniffer.php

@@ -0,0 +1,65 @@
+<?php
+
+namespace BookStack\Util;
+
+use finfo;
+
+/**
+ * Helper class to sniff out the mime-type of content resulting in
+ * a mime-type that's relatively safe to serve to a browser.
+ */
+class WebSafeMimeSniffer
+{
+
+    /**
+     * @var string[]
+     */
+    protected $safeMimes = [
+        'application/json',
+        'application/octet-stream',
+        'application/pdf',
+        'image/bmp',
+        'image/jpeg',
+        'image/png',
+        'image/gif',
+        'image/webp',
+        'image/avif',
+        'image/heic',
+        'text/css',
+        'text/csv',
+        'text/javascript',
+        'text/json',
+        'text/plain',
+        'video/x-msvideo',
+        'video/mp4',
+        'video/mpeg',
+        'video/ogg',
+        'video/webm',
+        'video/vp9',
+        'video/h264',
+        'video/av1',
+    ];
+
+    /**
+     * Sniff the mime-type from the given file content while running the result
+     * through an allow-list to ensure a web-safe result.
+     * Takes the content as a reference since the value may be quite large.
+     */
+    public function sniff(string &$content): string
+    {
+        $fInfo = new finfo(FILEINFO_MIME_TYPE);
+        $mime = $fInfo->buffer($content) ?: 'application/octet-stream';
+
+        if (in_array($mime, $this->safeMimes)) {
+            return $mime;
+        }
+
+        [$category] = explode('/', $mime, 2);
+        if ($category === 'text') {
+            return 'text/plain';
+        }
+
+        return 'application/octet-stream';
+    }
+
+}