For me,

I went to headers, deselected the original Content-type

Then went to presets, clicked manage presets

There, I clicked Add, then in the popup

a. Choose a Header Preset Name (anything will work)

b. For key -- enter Content-type

c. For value -- enter text/xml

d. Last click Add at bottom right corner

Close the manage preset popup

Click your new preset, then select the new Content-type at the bottom 

https://stackoverflow.com/a/63621370/10756812