z-index of custom file input

Created by: 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:

https://github.com/twbs/bootstrap/blob/06641ca0b3fe28021907f30c6f297d40f789f2fa/scss/_custom-forms.scss#L213-L218

https://github.com/twbs/bootstrap/blob/06641ca0b3fe28021907f30c6f297d40f789f2fa/scss/_custom-forms.scss#L236-L241