It looks like you have CSS Form Builder, so you can apply some form validation to check if the extension is correct.
To apply this validation, you would want to go to Insert->WebAssist->CSS Form Builder->Client Validation. This should open up an interface to add the validation. You would want to choose the form and the form field and then from the dropdown choose File Extension for the validation type. Then click the plus button and add the validation.
I'm not sure when the fix for the PNG-24 will be ready, so you may want to go with this workaround for now.