By default, language detection is disabled (false) as it could come with a cost. This default value can be changed by setting the index.mapping.attachment.detect_language setting. It can also be provided on a per document indexed using the _detect_language parameter.

Note that you can force language using _language field when sending your actual document:

PUT /test/person/1
{
    "my_attachment" : {
        "_language" : "en",
        "_content" : "... base64 encoded attachment ..."
    }
}