RFC 7231 suggests that a request with a missing Accept header should be treated as */*, but hitting the API POST endpoint for a data explorer query without said header returns 406 Not Acceptable. I’m not sure if this is specific to data explorer or if it applies to other POSTs, too. All the other GETs I’ve used haven’t had this issue.
It so happens that curl and Python’s requests append Accept: */* to all their requests by default, but not all tools do. You can see this behavior by disabling curl’s default with an empty header, e.g.,
curl -X POST "https://discourse.example.com/admin/plugins/explorer/queries/26/run" \
-H "Content-Type: multipart/form-data;" \
-H "Api-Key: <your_all_users_api_key>" \
-H "Api-Username: system" \
-H "Accept:"
Not a major thing, but it might be something that other tools hit, too… and it seems like it could be a simple fix, especially since all other endpoints I’ve used work without it.