added predefined mime types list and other minor ui improvements

ui/src/scss/_tooltip.scss

@@ -19,7 +19,8 @@
     transition: opacity var(--baseAnimationSpeed),
                 visibility var(--baseAnimationSpeed),
                 transform var(--baseAnimationSpeed);
-    transform: scale(0.98);
+    transform: translateY(1px);
+    backface-visibility: hidden;
     white-space: pre-line;
     word-break: break-word;
     @include hide();