try
@font-face {
font-family: 'icomoon';
src: asset-url('icomoon.eot');
src: asset-url('icomoon.eot?#iefix') format('embedded-opentype'),
asset-url('icomoon.ttf') format('truetype'),
asset-url('icomoon.woff') format('woff'),
asset-url('icomoon.svg#icomoon') format('svg');
font-weight: normal;
font-style: normal;
}
and rename your file to application.css.scss