Fix: thumbnail worker payload for non-xlsx files and change timeout to interval for thumbnail updates

backend/workers/thumbnails.go

@@ -167,12 +167,15 @@ func generateOfficeThumbnail(filePath, destPath string, cfg *config.Config) erro
 			"width":  1024,
 			"height": 1024,
 		},
-		"spreadsheetLayout": map[string]interface{}{
+	}
+
+	if ext == "xlsx" {
+		payload["spreadsheetLayout"] = map[string]interface{}{
 			"ignorePrintArea": true,
 			"printGridlines":  true,
 			"printHeadings":   true,
 			"fitToWidth":      1,
-		},
+		}
 	}
 
 	payloadBytes, err := json.Marshal(payload)