In the Modeler (since you’re on Mx6, open the properties dialog box for this File Manager. You’ll find the allowed extensions in the field highlighted below:
If you need to upload other extensions, you can modify that field.
You will have to turn your cancel button into a microflow that performs the cancel action and closes the page. When you have done that, you can set the “Abort on Validation Errors to: NO” in the settings of the microflow