Why file field button disappeared while uploading file?
Don't set upload_max_filesize larger than post_max_size if you are not using any library that takes care for such cases. Plupload integration with FileField Sources Plupload can get it managed for you.

Currently you are confusing your Drupal by allowing file upload larger than allowed data size. Given that data posted is file size + metadata size, both always equal to or greater than 0, it makes no sense and Drupal may fail.

Specifically, button disappeared because Form or Field API crashed, and it crashed because you mismatched sizes without providing appropriate handler.