os-nginx : Add error page file extension support

[Nemric]

resolve #4684 Allow users to change the extension of the error file so we can now set it to i.e. json to make it well read for app that require json These changes were made on a running Opnsense so perhaps I did miss to report some modifications here, so, please, review it carefully ^^ As I don't know Opnsense code at all I did a "just working" change see you :)

[kulikov-a]

Hi! maybe I would stick with the help style of the form (something like "Change the file extension if necessary. Response MIME type will be set according to the extension."). The rest is looking good to me

[Nemric]

Yeah, of course help message could be changed up to you ^^