How can I restrict file extensions in my Upload field?
You can specified the allowed extensions in the File Upload element. Go to the Advanced tab of the form, and in this field:
Allowed file extensions
you can see that the default allowed extensions are:
zip,rar,pdf,doc,xls,ppt,jpg,jpeg,gif,png
You can modify that as you wish. Just note that you should NOT put a dot (.) before the extension name.