Chatter REST API "/connect/files/users/me" resource to post single file.

ChatterApiPostSingleFile.java

String header = '--some_random_boundary_string' + '\r\n';
String separator = '\r\n' + '--some_random_boundary_string' + '\r\n';
String footer = '\r\n' + '\r\n' + '--some_random_boundary_string--';

HttpRequest req = new HttpRequest();
req.setEndpoint( URL.getSalesforceBaseUrl().toExternalForm() + '/services/data/v39.0/connect/files/users/me');
req.setHeader('Authorization', 'OAuth ' + UserInfo.getSessionId());
req.setHeader('Content-Type', 'multipart/form-data; boundary=some_random_boundary_string');
req.setHeader('Accept', 'application/json');
req.setBody(header +
            'Content-Disposition: form-data; name="fileData"; filename="FileA.txt"' + '\r\n' +
            'Content-Type: application/octet-stream; charset=ISO-8859-1' + '\r\n' + '\r\n' +
            'aaa' +
            footer
);
req.setMethod('POST');
req.setTimeout(60000);

HttpResponse res = new Http().send( req );

System.debug( res );
System.debug( res.getBody() );