wp-calypso/client/my-sites/media-library/upload-button.scss

.media-library__upload-button {
	position: relative;
	display: inline-block;
}

.media-library__upload-button-input {
	position: absolute;
	top: 0;
	right: 0;
	bottom: 0;
	left: 0;
	opacity: 0;
	cursor: pointer;

	&::-webkit-file-upload-button {
		cursor: pointer; /* webkit needs this to change cursor for file type input. */
	}
}

@include breakpoint-deprecated( "<480px" ) {
	.media-library__upload-button {
		margin-left: 0;
	}
}