@silverpill It only differs from the other platforms in requiring application/json, and it should have been working on mitra already but I didn't specify the profile field limit in the platform descriptor (defaults 0). Actually I also read it from the pleroma instance metadata, be cool to have that, looks like this
"fields_limits" : {
"name_length" : 512,
"value_length" : 2048,
"max_fields" : 10,
"max_remote_fields" : 20
},