Hi Bruno,
Thanks for using Syncfusion product.
In our Outlook demo, we are using both ToolStripSplitButtonEx and ToolStripDropDownButton in which arrows of ToolStripSplitButtonEx appears down. We have modified the arrow position of ToolStripDropDownButton to down for “ImageAboveText” scenario and these changes will be included in our Volume 4, 2018 release which is expected to be rolled out by next week. As of now, we suggest you to use ToolStripSplitButtonEx in your application to achieve your requirement. Please refer the sample and screenshot in below location.
Screenshot:
Arulraj A