diff --git a/backend/src/utils/files.ts b/backend/src/utils/files.ts
new file mode 100644
index 0000000..9e91d6d
--- /dev/null
+++ b/backend/src/utils/files.ts
@@ -0,0 +1,39 @@
+import path from 'node:path';
+import { env } from '../config/env.js';
+import type { FileRecord } from '../types.js';
+import { getPreviewMeta } from './preview.js';
+import { downloadUrl as gdriveDownloadUrl } from './gdrive.js';
+
+export function sanitizeFilename(name: string) {
+  const base = path.basename(name || 'file');
+  const cleaned = base
+    // eslint-disable-next-line no-control-regex
+    .replace(/[\x00-\x1f\x7f]/g, '')   // strip control characters
+    .replace(/[^\w.()\s-]+/g, '-')
+    .replace(/\s+/g, ' ')
+    .trim();
+  return cleaned || 'file';
+}
+
+export function fileToApi(file: FileRecord) {
+  const shareLink = `${env.publicAppUrl.replace(/\/$/, '')}/preview/${file.linkId}`;
+  const preview = getPreviewMeta(file);
+  return {
+    id: file.id,
+    linkId: file.linkId,
+    name: file.originalName,
+    size: file.sizeBytes,
+    type: file.mimeType,
+    uploader: file.uploader,
+    uploadedAt: file.createdAt,
+    expiresAt: file.expiresAt,
+    views: file.views,
+    downloads: file.downloads,
+    link: shareLink,
+    downloadUrl: gdriveDownloadUrl(file.gdriveId),
+    preview: {
+      ...preview,
+      url: preview.available ? `/api/v1/links/${file.linkId}/preview` : null,
+    },
+  };
+}