Excel supports ActiveX controls and Form Control type controls in Developer Tools ribbon -> Controls -> Insert. This meta bug is about supporting the Form Control type of controls, recognizable by the presence of xl\ctrlProps\ctrlPropN.xml files in the xlsx.
Some good example documents for future bug hunting are: attachment 127852 [details] from bug 103048 attachment 127728 [details] from bug 100374 attachment 156990 [details] from bug 129861 attachment 165013 [details] from bug 136397 attachment 173557 [details] from bug 143328