store icons in class

apps/files/css/files.css

@@ -85,10 +85,6 @@
 	background-color: rgb(179, 230, 255)!important;
 }
 
-.dropping-to-dir .thumbnail {
-	background-image: url(/nextcloud/core/img/filetypes/folder-drag-accept.svg)!important;
-}
-
 /* icons for sidebar */
 .nav-icon-files {
 	background-image: url('../img/folder.svg');

apps/files/js/file-upload.js

@@ -667,7 +667,7 @@ OC.Upload = {
 						OC.Upload._hideProgressBar();
 					}
 				});
-				fileupload.on('fileuploaddragover', function(e, data){
+				fileupload.on('fileuploaddragover', function(e){
 					$('#app-content').addClass('file-drag');
 
 					var filerow = $(e.delegatedEvent.target).closest('tr');
@@ -675,17 +675,20 @@ OC.Upload = {
 					if(!filerow.hasClass('dropping-to-dir')){
 						$('.dropping-to-dir').removeClass('dropping-to-dir');
 						$('.dir-drop').removeClass('dir-drop');
+						$('.icon-filetype-folder-drag-accept').removeClass('icon-filetype-folder-drag-accept');
 					}
 
 					if(filerow.attr('data-type') === 'dir'){
 						$('#app-content').addClass('dir-drop');
 						filerow.addClass('dropping-to-dir');
+						filerow.find('.thumbnail').addClass('icon-filetype-folder-drag-accept');
 					}
 				});
-				fileupload.on('fileuploaddragleave fileuploaddrop', function (e, data){
+				fileupload.on('fileuploaddragleave fileuploaddrop', function (){
 					$('#app-content').removeClass('file-drag');
 					$('.dropping-to-dir').removeClass('dropping-to-dir');
 					$('.dir-drop').removeClass('dir-drop');
+					$('.icon-filetype-folder-drag-accept').removeClass('icon-filetype-folder-drag-accept');
 				});
 			} else {
 				// for all browsers that don't support the progress bar

core/css/icons.css

@@ -353,6 +353,10 @@ img.icon-loading-small-dark, object.icon-loading-small-dark, video.icon-loading-
 	background-image: url('../img/filetypes/folder.svg');
 }
 
+.icon-filetype-folder-drag-accept {
+    background-image: url('../img/filetypes/folder-drag-accept.svg')!important;
+}
+
 .icon-home {
 	background-image: url('../img/places/home.svg');
 }