choices.cs

// LINQ all the things
bool FilesValid(HttpFileCollection files, IEnumerable<String> allowedExtensions)
{
  return files.AllKeys.Select(s => files[s])
    .Any(fi => !AllowedExtensions.Any(s => fi.FileName.EndsWith(s)));
}

// LINQ some of the things
bool FilesValid(HttpFileCollection files, IEnumerable<String> allowedExtensions)
{
  foreach (HttpPostedFile file in files.AllKeys.Select(s => files[s]))
  {
      if (!AllowedExtensions.Any(s => file.FileName.EndsWith(s)))
          return false;
  }

  return true;
}