how to send a file as a request as a raw http request body
As per documentation:
Name of the file to send. If left blank, JMeter does not send a file, if filled in, JMeter automatically sends the request as a multipart form request. If it is a POST or PUT or PATCH request and there is a single file whose 'Parameter name' attribute (below) is omitted, then the file is sent as the entire body of the request, i.e. no wrappers are added. This allows arbitrary bodies to be sent. This functionality is present for POST requests after version 2.2, and also for PUT requests after version 2.3. See below for some further information on parameter handling.
You can use the Jmeter function __fileToString for that:https://jmeter.apache.org/usermanual/functions.html#__FileToString