doc: add example for attach with custom layer media type

Signed-off-by: Billy Zha <[email protected]>

cmd/oras/root/attach.go

@@ -45,15 +45,18 @@ type attachOptions struct {
 func attachCmd() *cobra.Command {
 	var opts attachOptions
 	cmd := &cobra.Command{
-		Use:   "attach [flags] --artifact-type=<type> <name>{:<tag>|@<digest>} <file>[:<type>] [...]",
+		Use:   "attach [flags] --artifact-type=<type> <name>{:<tag>|@<digest>} <file>[:<layer_type>] [...]",
 		Short: "[Preview] Attach files to an existing artifact",
 		Long: `[Preview] Attach files to an existing artifact
 
 ** This command is in preview and under development. **
 
-Example - Attach file 'hi.txt' with type 'doc/example' to manifest 'hello:v1' in registry 'localhost:5000':
+Example - Attach file 'hi.txt' with aritifact type 'doc/example' to manifest 'hello:v1' in registry 'localhost:5000':
   oras attach --artifact-type doc/example localhost:5000/hello:v1 hi.txt
 
+Example - Push file "hi.txt" with the custom layer media type 'application/vnd.me.hi':
+  oras attach --artifact-type doc/example localhost:5000/hello:v1 hi.txt:application/vnd.me.hi
+
 Example - Attach file "hi.txt" using a specific method for the Referrers API:
   oras attach --artifact-type doc/example --distribution-spec v1.1-referrers-api localhost:5000/hello:v1 hi.txt # via API
   oras attach --artifact-type doc/example --distribution-spec v1.1-referrers-tag localhost:5000/hello:v1 hi.txt # via tag scheme