z-index of custom file input

[ysds]

Custom file inputs has z-index: 5 and 6. I'm not sure why 1 to 4 are not being used, but I think we can reduce the z-index.

Related code:

bootstrap/scss/_custom-forms.scss

Lines 213 to 218 in 06641ca

	.custom-file-control {
	position: absolute;
	top: 0;
	right: 0;
	left: 0;
	z-index: 5;

bootstrap/scss/_custom-forms.scss

Lines 236 to 241 in 06641ca

	&::before {
	position: absolute;
	top: -$custom-file-border-width;
	right: -$custom-file-border-width;
	bottom: -$custom-file-border-width;
	z-index: 6;