In order to setup allowed Upload File Extension in CformsII one need to perform below mentioned steps:
1) Login into wordpress admin area.
2) Click on CformsII -> Form settings.
3) Select the form for which one need to setup the allowed extensions.
4) Click on File Upload settings.
5) Apply the extension name in “Allowed File Extensions”. [Like, zip, RAR]
6) Click on update settings to apply the changes.