Update interfaces/FileAPI.idl

Source: https://github.com/tidoust/reffy-reports/blob/a1fde6b/whatwg/idl/FileAPI.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/519841411
diff --git a/interfaces/FileAPI.idl b/interfaces/FileAPI.idl
index 15b2e55..b5b4029 100644
--- a/interfaces/FileAPI.idl
+++ b/interfaces/FileAPI.idl
@@ -15,6 +15,11 @@
   Blob slice(optional [Clamp] long long start,
             optional [Clamp] long long end,
             optional DOMString contentType);
+
+  // read from the Blob.
+  [NewObject] ReadableStream stream();
+  [NewObject] Promise<USVString> text();
+  [NewObject] Promise<ArrayBuffer> arrayBuffer();
 };
 
 enum EndingType { "transparent", "native" };