Modifying proxy.pathsuffix to trim excess

[#1]

Our backend service does not allow .xml or .json to be passed but we want to allow a front-end user to be able to specify their desired response format in this way.

If a user tries to hit GET {base-URL-stuff}/books.xml then that means they want all books returned in XML format. However, since our backend service cannot handle the .xml we need to only forward the request as /books (essentially just trim the .xml off of proxy.pathsuffix).

So far I have tried to add in a .JS policy that is executed on conditional flow if the proxy.pathsuffix = "/*.xml", this executes but I haven't been able to properly set the proxy.pathsuffix to the trimmed version (without the .xml). Whenever I do a trace the all is still sent to the backend as /books.xml which causes a 400 error. Below is the .js code, the trace for this policy always shows a "=" with a slash through it for proxy.pathsuffix and request.url so as to say that it cannot be set in this way.

Does anyone have a way to make this work?

var suffix = context.getVariable("proxy.pathsuffix");
var suffix2 = suffix.slice(-4);
var suffix3 = suffix.slice(0, -4);

try {
    if(suffix2 == ".xml"){
        var req_verb = context.getVariable('request.verb');
        var req_scheme = context.getVariable('client.scheme');
        var req_host = context.getVariable('request.header.host');
        var base_path = context.getVariable('proxy.basepath');
        context.setVariable('request.uri', suffix3);
        var req_request_uri = context.getVariable('request.uri');
        var req_url = req_scheme + "://" + req_host + base_path + req_request_uri;
        context.setVariable('request.url', req_url);
        print("Req url: " + req_url);
    }
}
catch(e){}