Invalid File Issue

If you take a look at the error_log, you'll probably find an answer why is it happening.
I believe that the problem is in php.ini settings.

Create a simple php script with the following code and open it in your browser:

<?php
phpinfo();

Check the following values:
- upload_max_filesize
- post_max_size
- memory_limit (make sure that memory_limit > post_max_size)
and additionally, the following setting may cause problems as well:
- max_input_time

Simply increase all values in php.ini, restart your web server and it should work.