Ignore_malformed fails with file input plugin

[atira (Attila Boncok)]

I have a field called bytes_response that can take two types of values:

1. integer
2. -

Here's the mapping in ES (version 5.6.8):

"bytes_response": {
    "type": "integer",
	"ignore_malformed": true
}

Still I get the following error:

org.elasticsearch.index.mapper.MapperParsingException: failed to parse [bytes_response]
...
Caused by: java.lang.NumberFormatException: For input string: "-"

What's interesting is that when I used Filebeat to read the files and feed the events to Logstash via the beat input plugin, I had no such issues.
Current Logstash config uses the file input plugin since the files are local. Here's the config:

input {
        file {
                path => "/somepath/**/*.log"
                sincedb_path => "/somepath/sincedb"
                start_position => "beginning"
        }
}
filter {
        fingerprint {
                method => "SHA256"
                key => "somekey"
        }
        grok {
                patterns_dir => ["somepath"]
                match => [
                        "message" , "%{PREREQUEST} \"%{REQUEST}\" %{POSTREQUEST}"
                ]
                remove_field => ["message"]
        }
        if [request_path] and [http_referer] =~ "somestring" {
                date {
                        match => [ "timestamp", "dd/MMM/yyyy:HH:mm:ss Z" ]
                        remove_field => ["timestamp"] 
                }
                geoip {
                        source => "client_ip"
                        tag_on_failure => "internal_ip"
                }
        } else {
                drop { }
        }
}
output {
        elasticsearch {
                hosts => [ "localhost:9200" ]
                index => "someindex_%{+YYYY.MM}"
                document_id => "%{fingerprint}"
                action => "create"
        }
}

This is the Grok filter part that produces the field bytes_response:

(?<bytes_response>%{INT}|-)