diff --git a/TileFormats/Batched3DModel/README.md b/TileFormats/Batched3DModel/README.md
index dac79db46..03b13a2a4 100644
--- a/TileFormats/Batched3DModel/README.md
+++ b/TileFormats/Batched3DModel/README.md
@@ -99,6 +99,8 @@ Although not strictly required, clients may find the glTF [CESIUM_RTC](https://g
 
 `.b3dm`
 
+The file extension is optional. Valid implementations ignore it and identify a content's format by the `magic` field in its header.
+
 ## MIME Type
 
 _TODO, [#60](https://github.com/AnalyticalGraphicsInc/3d-tiles/issues/60)_
diff --git a/TileFormats/Composite/README.md b/TileFormats/Composite/README.md
index 4329156c2..8a98d9e18 100644
--- a/TileFormats/Composite/README.md
+++ b/TileFormats/Composite/README.md
@@ -54,6 +54,8 @@ Refer to the spec for each tile format for more details.
 
 `.cmpt`
 
+The file extension is optional. Valid implementations ignore it and identify a content's format by the `magic` field in its header.
+
 ## MIME Type
 
 _TODO, [#60](https://github.com/AnalyticalGraphicsInc/3d-tiles/issues/60)_
diff --git a/TileFormats/Instanced3DModel/README.md b/TileFormats/Instanced3DModel/README.md
index 6874f1b75..49ec0c553 100644
--- a/TileFormats/Instanced3DModel/README.md
+++ b/TileFormats/Instanced3DModel/README.md
@@ -246,6 +246,8 @@ If the glTF asset is embedded, it must be 8-byte aligned so that glTF's byte-ali
 
 `.i3dm`
 
+The file extension is optional. Valid implementations ignore it and identify a content's format by the `magic` field in its header.
+
 ## MIME Type
 
 _TODO, [#60](https://github.com/AnalyticalGraphicsInc/3d-tiles/issues/60)_
diff --git a/TileFormats/PointCloud/README.md b/TileFormats/PointCloud/README.md
index 2493b2839..c579f1a14 100644
--- a/TileFormats/PointCloud/README.md
+++ b/TileFormats/PointCloud/README.md
@@ -298,6 +298,8 @@ See the [Batch Table](../BatchTable/README.md) reference for more information.
 
 `.pnts`
 
+The file extension is optional. Valid implementations ignore it and identify a content's format by the `magic` field in its header.
+
 ## MIME Type
 
 _TODO, [#60](https://github.com/AnalyticalGraphicsInc/3d-tiles/issues/60)_