The file input field creates a simple text input for upload a single file. You are able to select a file from the Media Lirary or enter file URL directly (even URL for a file hosted on another website).
This field doesn’t have any specific settings. It only uses common field settings.
This field saves file URL in the database.
To get field value (file URL), use the following code:
$value = rwmb_meta( $field_id ); echo $value;
If you want to show the file as an image, use the following code:
$value = rwmb_meta( $field_id ); echo '<img src="', $value, '">';
Read more about rwmb_meta().