AJAX is OFF (It will change page after submission).
You can enabled ajax on from flexi settings. Which will not change the page and submission starts on same page.
Shortcode used:
[flexi-form class="pure-form pure-form-stacked" title="My Form" name="my-form" ajax="false" type="image"][flexi-form-tag type="post_title" title="Title" value="" placeholder="" required="true"][flexi-form-tag type="file_multiple" title="Select File" class="flexi_drag_file" multiple="true" required="true"][flexi-form-tag type="submit" name="flexi_submit_button" value="Submit"][/flexi-form]