To explicitly omit something from being sent in a request, use omit.
# as the default `Content-Type` header is `application/json` that will be sent
client.post("/upload/files", files={"file": b"my raw file content"})
# you can't explicitly override the header as it has to be dynamically generated
# to look something like: 'multipart/form-data; boundary=0d8382fcf5f8c3be01ca2e11002d2983'
client.post(..., headers={"Content-Type": "multipart/form-data"})
# instead you can remove the default `application/json` header by passing omit
client.post(..., headers={"Content-Type": omit})