Angular POST multipart/form-data with application/json & text/csv
Faced the same issue.Try using blob and You can set the type in the blob as application/json. You can initialize the blob with your values and it worked for me.
If you make a "multipart/form-data" request, then automatically, the "application/JSON" header will be left out. You cannot have both in the same request. The two post requests are mutually exclusive.