[TIMOB-16712] Mobile Web: App fails to load on Windows Phone 8 (IE10) due to Ti.Filesystem calling unsupported xhr.overrideMimeType()

Description

Posting a local file from an application using the following code sample results in an error.

Steps To Reproduce

var url = 'http://httpbin.org/post';
var client = Ti.Network.createHTTPClient({
    onload : function(e) {
        alert("SUCCESS");
    },
    onerror : function(e) {
        alert("FAIL");
    },
    timeout:5000
});
client.open('POST', url);
client.send({files:Ti.Filesystem.getFile('textfile.txt').read()});

Add a text file called 'textfile.txt' in your resources folder.

Expected Result

Should alert the user saying "SUCCESS"

Actual Result

Throws error saying "Object doesn't support property or method 'overrideMimeType' at x-wmapp0:index.html (line 4030)"