r1xnx's post in KESL Container REST-API detects nothing was marked as the answer
The issue must have been cURL. I have done everything in Python Requests, which works:
>>> import requests
>>> requests.post("http://127.0.0.1:8085/scans?wait=1", headers={'content-type': 'application/octet-stream'}, data=rb"X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*").json()
{'completed': '2023-01-25T15:19:55.179518+00:00', 'created': '2023-01-25T15:19:52.343156+00:00', 'progress': 100, 'scan_result': {'noname': {'started': '2023-01-25T15:19:53+00:00', 'stopped': '2023-01-25T15:19:54+00:00', 'threats': [{'name': 'EICAR-Test-File', 'object': '/root/kesl-service/tmp/21729d49-4986-4469-b017-90d2e92c34c3'}], 'verdict': 'infected'}}, 'status': 'completed', 'verdicts': ['infected']}
