Fix: Hide the download button if it is an empty file #3762 (#3853)

### What problem does this PR solve?

Fix: Hide the download button if it is an empty file #3762

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/constants/knowledge.ts

@@ -57,3 +57,8 @@ export enum KnowledgeSearchParams {
   DocumentId = 'doc_id',
   KnowledgeId = 'id',
 }
+
+export enum DocumentType {
+  Virtual = 'virtual',
+  Visual = 'visual',
+}