Add link to compression decisions

Volume-Server-API.md

@@ -29,7 +29,7 @@ curl -F file=@/home/chris/myphoto.jpg http://127.0.0.1:8080/3,01637037d6
 {"size": 43234}
 ```
 
-The size returned is the size stored on SeaweedFS, sometimes the file is automatically gzipped based on the mime type.
+The size returned is the size stored on SeaweedFS, sometimes the file is automatically gzipped based on the file extension or mime type [(see when compression will be applied automatically)](https://github.com/chrislusf/seaweedfs/blob/c42b95c596f762dcca2bc9c7e7a918ab8ca8b206/weed/util/compression.go#L111).
 
 | URL Parameter | Description | Default |
 | ---- | -- | -- |