diff --git a/Filer-Server-API.md b/Filer-Server-API.md
index 8cbf220..fced80b 100644
--- a/Filer-Server-API.md
+++ b/Filer-Server-API.md
@@ -36,7 +36,8 @@ You can append to any HTTP API with &pretty=y to see a formatted json output.
 | ttl | time to live, examples, 3m: 3 minutes, 4h: 4 hours, 5d: 5 days, 6w: 6 weeks, 7M: 7 months, 8y: 8 years | empty |
 | maxMB | max chunk size | empty |
 | mode | file mode | 0660 |
-| op | file operation, currently only support "append" | empty |
+| offset | incompatible with `op=append`. Defines the number of bytes from the file beginning to insert the uploaded chunk | empty |
+| op | file operation, currently only support "append", incompatible with `offset=`. | empty |
 | skipCheckParentDir | Ensuring parent directory exists cost one metadata API call. Skipping this can reduce network latency. | false |
 | header: `Content-Type` | used for auto compression | empty |
 | header: `Content-Disposition` | used as response content-disposition | empty |