Is there a way to set the default output type header in AXIS2 say for example to use "Content: application/json" even if the request does not have a content type header sent to the service?
You could also use a content-type property mediator in each sequence, which can assign the content type. Here is an example:
<property name="ContentType" value="text/xml" scope="axis2"/>
Here is documentation on the Axis2 properties [1]. If you wish this to be applied by default, it would depend on what product you're using. With the api manager, you may edit the velocity-template.xml which is located in [apim home]/repository/resources/api-templates/. Any changes to this template file will affect all APIs as they are created or republished.
[1] https://docs.wso2.com/display/ESB481/Generic+Properties
Cheers, Colin