if (File.Exists(fileName) ||
    File.Exists(Directory.GetParent(Path.GetDirectoryName(fileName)).FullName +
        Path.DirectorySeperatorChar +
        Path.GetFileName(fileName)))

var fileName = @"c:tempfoo.txt";

var fileExists =
    File.Exists(fileName) ||
    File.Exists(
        Path.Combine(
            Directory.GetParent(Path.GetDirectoryName(fileName)).FullName,
            Path.GetFileName(fileName)
        )
    );

static IEnumerable<string> GetFileSearchPaths(string fileName)
{
    yield return fileName;
    yield return Path.Combine(
        Directory.GetParent(Path.GetDirectoryName(fileName)).FullName,
        Path.GetFileName(fileName)
    );
}

// or as an extension
static bool FileExists(string fileName)
{
    return GetFileSearchPaths(fileName).Any(File.Exists);
}

if(FileExists(fileName)) ...

static string FindFile(this string fileName)
{
    return GetFileSearchPaths(fileName).FirstOrDefault(x => File.Exists(x));
}

var actualFileName = FindFile(fileName);

if (!string.IsNullOrEmpty(actualFileName)) ...

MessageLabel.Text = ac.CheckExistingFile("~/Files/Articles/Article1.txt");