diff --git a/docs/src/extensions.md b/docs/src/extensions.md
index 3e84f70d80..7aaf9db14f 100644
--- a/docs/src/extensions.md
+++ b/docs/src/extensions.md
@@ -43,3 +43,10 @@ therefore welcomed.
 
 `cut` can separate fields by whitespace (Space and Tab) with `-w` flag. This
 feature is adopted from [FreeBSD](https://www.freebsd.org/cgi/man.cgi?cut).
+
+## `fmt`
+
+`fmt` has additional flags for prefixes: `-P/--skip-prefix`, `-x/--exact-prefix`, and
+`-X/--exact-skip-prefix`. With `-m/--preserve-headers`, an attempt is made to detect and preserve
+mail headers in the input. `-q/--quick` breaks lines more quickly. And `-T/--tab-width` defines the
+number of spaces representing a tab when determining the line length.