Fix mixins/extends confusion

sass/elements/form.sass

@@ -151,7 +151,8 @@ $help-size: $size-small !default
     height: 2.25em
   &:not(.is-multiple):not(.is-loading)
     &::after
-      +arrow($input-arrow)
+      @extend %arrow
+      border-color: $input-arrow
       right: 1.125em
       z-index: 4
   &.is-rounded
@@ -213,7 +214,7 @@ $help-size: $size-small !default
       width: 100%
   &.is-loading
     &::after
-      +loader
+      @extend %loader
       margin-top: 0
       position: absolute
       right: 0.625em
@@ -227,7 +228,7 @@ $help-size: $size-small !default
       font-size: $size-large
 
 .file
-  +unselectable
+  @extend %unselectable
   align-items: stretch
   display: flex
   justify-content: flex-start
@@ -614,7 +615,7 @@ $help-size: $size-small !default
       right: 0
   &.is-loading
     &::after
-      +loader
+      @extend %loader
       position: absolute !important
       right: 0.625em
       top: 0.625em