Cannot load the registered search template file

I create a search template as follows and save it to /config/scripts as mytemplate.mustache

{
"template": {
"query": {
"terms": {
"Keywords": [
"{{#Keywords}}",
"{{.}}",
"{{/Keywords}}"
]
}
}
}
}

Then tried to use the template in query:
GET /_search/template
{
"template": {
"file" : "mytemplate.mustache"
},
"params": {
"Keywords": [ "office", "xbox" ]
}

But got following error:
ElasticsearchIllegalArgumentException[Unable to find on disk script mytemplate.mustache]

What steps was I missing?
}