修复txt导出不映射的问题

electron/services/exportService.ts

@@ -2546,7 +2546,10 @@ class ExportService {
 
       for (let i = 0; i < sortedMessages.length; i++) {
         const msg = sortedMessages[i]
-        const contentValue = this.formatPlainExportContent(
+        const mediaKey = `${msg.localType}_${msg.localId}`
+        const mediaItem = mediaCache.get(mediaKey)
+        const contentValue = mediaItem?.relativePath
+          || this.formatPlainExportContent(
             msg.content,
             msg.localType,
             options,