Importing text from text files and merging to existing documents

I have an existing elastic engine containing documents with an ID and some metadata (date, original file path, author etc.).

I now have the text files which have the extracted text for these documents. The text files are "<document_id>.txt" all sitting in a folder.

The configuration file then has:

intput {
file{
path => "/FileData/Sourcefolder/*.txt
start_position => "beginning"
sincedb_path => "/dev/null"
}
}
output {
action => update
(url, api_key, engine etc. as used for the first upload)
}

Is there a way I can reference the file name of the input file? I want something like:
"document_id" => fileName
"full_text" => fileText

Yes, a file input adds a [path] field to the event.