123FormBuilder is a free online form builder with file upload functionality. Users can collect files from their respondents through form submissions by adding file upload fields to the form.
All supported file extensions of the File Upload field are located in the Validation section, within the field’s options. To allow only certain file extensions upon upload, at File types accepted use the drop down list to check the extensions you want to use.
If you need a new file extension, simply click on Add.