Here show that You can locat doc information from API
The following is the information from this API, which I think is enough for you.
A custom Result type for sending raw data (via an InputStream) directly to the HttpServletResponse. Very useful for allowing users to download content.
This result type takes the following parameters:
contentType - the stream mime-type as sent to the web browser (default = text/plain).
contentLength - the stream length in bytes (the browser displays a progress bar).
contentDisposition - the content disposition header value for specifing the file name (default = inline, values are typically attachment;filename="document.pdf".
inputName - the name of the InputStream property from the chained action (default = inputStream).
bufferSize - the size of the buffer to copy from input to output (default = 1024).
allowCaching if set to 'false' it will set the headers 'Pragma' and 'Cache-Control' to 'no-cahce', and prevent client from caching the content. (default = true)
contentCharSet if set to a string, ';charset=value' will be added to the content-type header, where value is the string set. If set to an expression, the result of evaluating the expression will be used. If not set, then no charset will be set on the header
These parameters can also be set by exposing a similarly named getter method on your Action. For example, you can provide getContentType() to override that parameter for the current action.
if you do exactly as this statement, I think the problem will be sovled.