Update file input to remove height and line-height

This particular form control is wildly inaccurate across browsers, so
we're removing the `height` and `line-height`, but adding `display:
block` so it renders similarly to our other inputs.

less/forms.less

@@ -124,8 +124,7 @@ input[type="checkbox"] {
 
 // Set the height of select and file controls to match text inputs
 input[type="file"] {
-  height: @input-height-base; /* In IE7, the height of the select element cannot be changed by height, only font-size. TODO: Check if this is still needed when dropping IE7 support */
-  line-height: @input-height-base;
+  display: block;
 }
 
 // Make multiple select elements height not fixed