Let the content determine overflow.

core/css/jquery.ocdialog.css

@@ -19,7 +19,6 @@
 .oc-dialog-content {
 	z-index: 1000;
 	background: white;
-	overflow-y: auto;
 }
 .oc-dialog-separator {
 }

core/css/styles.css

@@ -393,6 +393,7 @@ a.bookmarklet { background-color:#ddd; border:1px solid #ccc; padding:5px;paddin
 #oc-dialog-filepicker-content .dirtree span:not(:last-child)::after { content: '>'; padding: 3px;}
 #oc-dialog-filepicker-content .filelist {
 	overflow-y:auto;
+	max-height: 300px;
 	background-color:white;
 	width:100%;
 }