Geoserver: Running a WPS process asynchronously returns GML format always

I want to run a WPS process async, I'm sending the following request:

<?xml version="1.0" encoding="UTF-8"?> <wps:Execute xmlns:wps="http://www.opengis.net/wps/1.0.0" xmlns="http://www.opengis.net/wps/1.0.0" xmlns:gml="http://www.opengis.net/gml" xmlns:ogc="http://www.opengis.net/ogc" xmlns:ows="http://www.opengis.net/ows/1.1" xmlns:wcs="http://www.opengis.net/wcs/1.1.1" xmlns:wfs="http://www.opengis.net/wfs" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1.0.0" service="WPS" xsi:schemaLocation="http://www.opengis.net/wps/1.0.0 http://schemas.opengis.net/wps/1.0.0/wpsAll.xsd"> <ows:Identifier>ras:Contour</ows:Identifier> <wps:DataInputs> <wps:Input> <ows:Identifier>data</ows:Identifier> <wps:Reference mimeType="image/tiff" xlink:href="http://geoserver/wcs" method="POST"> <wps:Body> <wcs:GetCoverage service="WCS" version="1.1.1"> <ows:Identifier>tst:elevations</ows:Identifier> <wcs:DomainSubset> <ows:BoundingBox crs="http://www.opengis.net/gml/srs/epsg.xml#4326"> <ows:LowerCorner>30.0 28.0</ows:LowerCorner> <ows:UpperCorner>30.2 28.2</ows:UpperCorner> </ows:BoundingBox> </wcs:DomainSubset> <wcs:Output format="image/tiff" /> </wcs:GetCoverage> </wps:Body> </wps:Reference> </wps:Input> <wps:Input> <ows:Identifier>interval</ows:Identifier> <wps:Data> <wps:LiteralData>100</wps:LiteralData> </wps:Data> </wps:Input> <wps:Input> <ows:Identifier>simplify</ows:Identifier> <wps:Data> <wps:LiteralData>1</wps:LiteralData> </wps:Data> </wps:Input> </wps:DataInputs> <wps:ResponseForm> <wps:ResponseDocument storeExecuteResponse="true" status="true"> <wps:RawDataOutput mimeType="application/json"> <ows:Identifier>result</ows:Identifier> </wps:RawDataOutput> </wps:ResponseDocument> </wps:ResponseForm> </wps:Execute> 

Then I use GetExecutionStatus to get the status and the result of the above request.

http://localhost:8080/geoserver/ows?service=WPS&version=1.0.0&request=GetExecutionStatus&executionId=7bf07277-62fe-4a7d-a4e3-dd2b19a5582e 

although I set the output mimetype to "application/json" as shown above, I got the result in GML format.

If I run the same request synchronously I get the result in json format.

What is the problem?

Here is a sample response:

<wps:ExecuteResponse xml:lang="en" service="WPS" serviceInstance="http://localhost:8080/geoserver/ows?" statusLocation="http://localhost:8080/geoserver/ows?service=WPS&version=1.0.0&request=GetExecutionStatus&executionId=d3f3a311-7576-464b-a683-7e13f76166b8" version="1.0.0"> <wps:Process wps:processVersion="1.0.0"> <ows:Identifier>ras:Contour</ows:Identifier> <ows:Title>Contour</ows:Title> <ows:Abstract>Computes contour lines at specified intervals or levels for the values in a raster.</ows:Abstract> </wps:Process> <wps:Status creationTime="2018-03-31T15:50:22.746Z"> <wps:ProcessSucceeded>Process succeeded.</wps:ProcessSucceeded> </wps:Status> <wps:ProcessOutputs> <wps:Output> <ows:Identifier>result</ows:Identifier> <ows:Title>Contour line features. Contour level is in value attribute.</ows:Title> <wps:Data> <wps:ComplexData mimeType="text/xml; subtype=wfs-collection/1.0"> <wfs:FeatureCollection> <gml:boundedBy> <gml:Box srsName="http://www.opengis.net/gml/srs/epsg.xml#4326"> <gml:coord><gml:X>30.00013888888889</gml:X><gml:Y>28.000138696255206</gml:Y></gml:coord> <gml:coord><gml:X>30.19986111111112</gml:X><gml:Y>28.19986130374481</gml:Y></gml:coord> </gml:Box> </gml:boundedBy> <gml:featureMember> <feature:geotiff_coverage fid="0"><gml:boundedBy> <gml:Box srsName="http://www.opengis.net/gml/srs/epsg.xml#4326"> <gml:coord><gml:X>30.003364197530864</gml:X><gml:Y>28.19595647071376</gml:Y></gml:coord> <gml:coord><gml:X>30.00625</gml:X><gml:Y>28.19986130374481</gml:Y></gml:coord> </gml:Box> </gml:boundedBy> <feature:the_geom> <gml:LineString> <gml:coordinates>30.00371913580247,28.19986130374481 30.00366452991454,28.19977595220315 30.003625283446716</gml:coordinates> </gml:LineString> </feature:the_geom>