### Fixed
- Modern file uploader: correctly process post_max_size value from php.ini (js should not send that file at all).
- Modern file uploader: make error message more clear when a file was not uploaded.
- Modern file uploader: when file is being uploaded do not change Submit button text.
- Modern file uploader: correctly process WordPress Media library integration and conditional logic.
- Modern file uploader: some servers don't have mime extension installed, so use WP function to determine mime type.
