Auth=md5 overriding in extensions

Is there a policy or a general option to indicate that all IAX2 extensions use auth=md5.? Because saving extensions overrides iax_additional.conf
Why auth is no contemplated in freepbx extension configuration?
Thanks in advance for your help.

Do not edit iax_additional.conf, It is a FreePBX generated file (yes every time you save and apply changes) and as it clearly states at the top of the file you are not supposed to edit it.

To learn what files are owned by FreePBX and should or should not be edited please see: