Hardware Inventory API Body Content

I am using a tool called ZappySys inside of SSIS to aytomate the pulling of data from the APIs.  

If I enter the hostIds and field variables directly into the body everything works fine, however this is not scalable.  I need to be able to programaticaly create a file to use in the Body that includes all of the hostIds I want to look at as well as the field I want to return.  I tried it with both a JSON file and a normal TXT file, but receive errors.   I get the same error in PostMan when i try to use a file rather than harcoding the values.  Has anyone succesfully utlized a file with this API?

Status Code: InternalServerError

Response Body: {"message":"Unexpected error: The incoming message has an unexpected message format 'Raw'. The expected message formats for the operation are 'Xml', 'Json'. This can be because a WebContentTypeMapper has not been configured on the binding. See the documentation of WebContentTypeMapper for more details.","ticket":"d698695384"}