Untitled

Using System.IO
 ... lost of code
privat void share()
....
    if (File.exist(filename)
    {
       add file to share
    }

StorageFile file;
try {
    file = await ApplicationData.Current.LocalStorage.GetFileAsync("foo.txt");
}
catch (FileNotFoundException) {
    file = null;
}

public static async Task<bool> FileExistsAsync(this StorageFolder folder, string fileName)
{
    try
    {
        await folder.GetFileAsync(fileName);
        return true;
    }
    catch (FileNotFoundException)
    {
        return false;
    }
}

var sourceFolder = Windows.ApplicationModel.Package.Current.InstalledLocation;
sourceFolder = await sourceFolder.GetFolderAsync("Assets");
var files = await sourceFolder.GetFilesAsync();
var requiredFiles = new List<String> { "ThisWorks.png", "NotHere.png" };
foreach(var filename in requiredFiles)
{
    // your example logic here...
    Debug.WriteLine(filename + " " + (files.Any(f => f.Name == filename) ? "Exists" : "Doesn't exist"));
}