diff --git a/backend/src/utils/preview.ts b/backend/src/utils/preview.ts
new file mode 100644
index 0000000..5c4c993
--- /dev/null
+++ b/backend/src/utils/preview.ts
@@ -0,0 +1,59 @@
+import { env } from '../config/env.js';
+import type { FileRecord } from '../types.js';
+
+export type PreviewKind = 'image' | 'video' | 'audio' | 'pdf' | 'text';
+
+const TEXT_MIME_TYPES = new Set([
+  'application/json',
+  'application/x-ndjson',
+  'text/csv',
+  'text/markdown',
+  'text/plain',
+]);
+
+const IMAGE_MIME_TYPES = new Set([
+  'image/avif',
+  'image/bmp',
+  'image/gif',
+  'image/jpeg',
+  'image/png',
+  'image/webp',
+]);
+
+const VIDEO_MIME_TYPES = new Set([
+  'video/mp4',
+  'video/ogg',
+  'video/webm',
+]);
+
+const AUDIO_MIME_TYPES = new Set([
+  'audio/aac',
+  'audio/flac',
+  'audio/mpeg',
+  'audio/mp4',
+  'audio/ogg',
+  'audio/wav',
+  'audio/webm',
+]);
+
+export function getPreviewKind(mimeType: string): PreviewKind | null {
+  const normalized = mimeType.toLowerCase().split(';')[0].trim();
+  if (IMAGE_MIME_TYPES.has(normalized)) return 'image';
+  if (VIDEO_MIME_TYPES.has(normalized)) return 'video';
+  if (AUDIO_MIME_TYPES.has(normalized)) return 'audio';
+  if (normalized === 'application/pdf') return 'pdf';
+  if (TEXT_MIME_TYPES.has(normalized)) return 'text';
+  return null;
+}
+
+export function getPreviewMeta(file: FileRecord) {
+  const kind = getPreviewKind(file.mimeType);
+  const available = Boolean(kind) && file.sizeBytes <= env.previewMaxFileSizeBytes;
+
+  return {
+    available,
+    kind: available ? kind : null,
+    maxSize: env.previewMaxFileSizeBytes,
+  };
+}
+