access all files in a folder

  var files = aqFileSystem.FindFiles("C:\\Excel Files", "*.xlsx");
  while (files.HasNext()) {
    file = files.Next();
    //Upload file 
  }

From your question, I'm assuming you don't need to actually read the contents of the file but simply get the list of files and then access them for upload.

In that case, I'd use the aqFileSystem.FindFiles method to return a list of files in a folder that match a particular search criteria.  You can then iterate through that list of files and obtain the full path for each file which you can then use for upload. There's an example on how to use the method in the article I linked.  Hope this helps!