how to hide the below-marked attachment file option from the form?

how to hide the below-marked attachment file option from the form?
I don't think there is an option for that, but you can apply JS/jQuery to hide it
Unfortunately I don't have a trial with SharePoint enabled here, if you inspect the button element you should probably find an ID or Class that should be a unique identifier, so you can hide it using that.. for example if it's by class:
$(".new-folder").hide();