fix(mobile): handle file picker cancellation

mobile-uni/src/services/knowledge.ts

@@ -10,6 +10,7 @@ import type {
 } from '@/types/api';
 import { apiRequest, apiUrl, authHeaders, readTextPayload } from './api';
 import { getSelectedPosition } from './auth';
+import { fileSelectionFailure } from '../utils/file-selection';
 
 type RawKnowledgeSnippet = KnowledgeSnippet & {
   title?: string;
@@ -44,7 +45,7 @@ export const chooseKnowledgeMedia = () =>
         }
         resolve({ path, name, size, kind });
       },
-      fail: () => reject(new Error('未选择附件'))
+      fail: (error) => reject(fileSelectionFailure(error, '附件'))
     });
   });