You could use the Fileopenpicker in your JS code like this:

var openPicker = new Windows.Storage.Pickers.FileOpenPicker();
    openPicker.viewMode = Windows.Storage.Pickers.PickerViewMode.thumbnail;
    openPicker.suggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.picturesLibrary;
    // Users expect to have a filtered view of their folders depending on the scenario.
    // For example, when choosing a documents folder, restrict the filetypes to documents for your application.
    openPicker.fileTypeFilter.replaceAll([".png", ".jpg", ".jpeg"]);

    // Open the picker for the user to pick a file
    openPicker.pickSingleFileAsync().then(function (file) {
        if (file) {
            // Application now has read/write access to the picked file
            WinJS.log && WinJS.log("Picked photo: " + file.name, "sample", "status");
        } else {
            // The picker was dismissed with no selected file
            WinJS.log && WinJS.log("Operation cancelled.", "sample", "status");
        }
    });

What you need is to change the file type to the target type you need.

There is a JS sample that you could directly refer to: FilePicker - JS.