Use server-verified mime_type for attachment thumbnail detection (#60)

renderAttachments() decided whether to render an image thumbnail by
regex-matching the display filename extension, rather than the
finfo-verified mime_type the API already returns. A file whose real
type differs from its display name (e.g. a PDF a user named
photo.png) rendered a broken <img> instead of falling back to the
file-type icon.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

assets/js/ticket.js

@@ -1048,7 +1048,7 @@ function renderAttachments(attachments) {
         });
         const uploadDate = `<span class="ts-cell" data-ts="${lt.escHtml(att.uploaded_at)}" title="${lt.escHtml(uploadDateFormatted)}">${lt.time.ago(att.uploaded_at)}</span>`;
 
-        const isImage = /\.(png|jpe?g|gif|webp|svg|bmp)$/i.test(att.original_filename);
+        const isImage = /^image\//i.test(att.mime_type || '');
         const imgUrl  = `/api/download_attachment.php?id=${att.attachment_id}&inline=1`;
         const iconHtml = isImage
             ? `<a href="${imgUrl}" class="lt-lightbox-trigger" data-lightbox="ticket-attachments" title="${lt.escHtml(att.original_filename)}">