@@ -32,6 +32,7 @@ mpt_api_client/
 │   ├── async_service.py     # AsyncService — async service (extends ServiceBase)
 │   ├── query_state.py       # Query parameter accumulation
 │   ├── client_utils.py      # URL validation helpers
+│   ├── file_utils.py        # Upload content-type declaration
 │   ├── types.py             # Type aliases (Response, HeaderTypes, etc.)
 │   ├── json_envelope_parser.py  # Incremental {$meta, data} envelope parsing
 │   ├── jsonl_lines.py       # JSONL record-line splitting (newlines only)
@@ -218,7 +219,9 @@ class ProductsService(
 - base URL resolution
 - retry transport (configurable)
 - error transformation into `MPTHttpError` / `MPTAPIError`
-- multipart file upload support
+- multipart file upload support; `http/file_utils.py` declares a content type for an
+  upload whose extension `mimetypes` does not recognise, such as the `.jsonl` a billing
+  journal carries, which httpx would otherwise send as `application/octet-stream`
 
 Transport-level settings (`base_url`, `timeout`, `retries`) are grouped in the
 `TransportSettings` dataclass (`http/transport_settings.py`), passed to the client
 
@@ -1,5 +1,6 @@
 APPLICATION_JSON = "application/json"
 APPLICATION_JSONL = "application/jsonl"
+APPLICATION_X_NDJSON = "application/x-ndjson"
 MIMETYPE_EXCEL_XLSX = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
 
 MPT_STREAMING_HEADER = "MPT-Streaming"
 
@@ -0,0 +1,46 @@
+from pathlib import Path
+
+from mpt_api_client.http.types import FileContent, FileTypes
+
+DEFAULT_UPLOAD_FILENAME = "upload"
+
+
+def _upload_filename(payload: FileContent) -> str:
+    """Derive the multipart filename the way httpx does for content passed without one.
+
+    Only a file object opened from a path carries a usable ``name``; ``bytes``, ``str`` and
+    ``io.BytesIO`` have none, and a file opened from a descriptor names an ``int``. httpx
+    falls back to ``upload`` in each of those cases, so this does too. The basename keeps an
+    absolute path out of the multipart part.
+    """
+    name = getattr(payload, "name", DEFAULT_UPLOAD_FILENAME)
+
+    return Path(str(name)).name or DEFAULT_UPLOAD_FILENAME
+
+
+def declare_content_type(file_part: FileTypes, content_type: str) -> FileTypes:
+    """Declare a content type on a file part unless the caller already declared one.
+
+    httpx derives the content type of an upload from the filename extension and falls back
+    to ``application/octet-stream`` whenever :mod:`mimetypes` does not recognise it. The
+    API rejects that fallback, so endpoints whose payload extension is unknown to
+    :mod:`mimetypes`, such as ``.jsonl``, have to name their own type.
+
+    Args:
+        file_part: File part in any of the shapes httpx accepts, which fix the content type
+            at the third position: bare content, ``(filename, content)``,
+            ``(filename, content, content_type)`` or
+            ``(filename, content, content_type, headers)``.
+        content_type: Content type to declare when the file part carries none.
+
+    Returns:
+        FileTypes: File part with a content type declared.
+    """
+    if not isinstance(file_part, tuple):
+        return _upload_filename(file_part), file_part, content_type
+
+    filename, payload, *type_and_headers = file_part
+    declared_type = type_and_headers[0] if type_and_headers else None
+    headers = type_and_headers[1:]
+
+    return (filename, payload, declared_type or content_type, *headers)
@@ -1,4 +1,6 @@
+from mpt_api_client.constants import APPLICATION_X_NDJSON
 from mpt_api_client.http import AsyncService, Service
+from mpt_api_client.http.file_utils import declare_content_type
 from mpt_api_client.http.mixins import (
     AsyncCollectionMixin,
     AsyncManagedResourceMixin,
@@ -52,6 +54,9 @@ def upload(
     ) -> Journal | ModelCollection[Journal]:  # noqa: WPS110
         """Upload journal file.
 
+        The journal file holds JSON Lines. Unless the caller declares a content type of its
+        own, the file is sent as ``application/x-ndjson``, which the endpoint accepts.
+
         Args:
             journal_id: Journal ID.
             file: journal file.
@@ -60,24 +65,22 @@ def upload(
             Journal | ModelCollection[Journal]: The uploaded resource as a single Journal
             instance, or a ModelCollection[Journal] when the response contains multiple records.
         """
-        files = {}
+        files: dict[str, FileTypes] = {}
 
         if file:
-            files[self._upload_file_key] = file  # UNUSED type: ignore[attr-defined]
-            files[self._upload_data_key] = journal_id  # UNUSED type: ignore
+            files[self._upload_file_key] = declare_content_type(file, APPLICATION_X_NDJSON)
+            files[self._upload_data_key] = (None, journal_id)
 
         path = join_url_path(self.path, journal_id, "upload")
 
-        response = self.http_client.request(  # UNUSED type: ignore[attr-defined]
+        response = self.http_client.request(
             "post",
-            path,  # UNUSED type: ignore[attr-defined]
+            path,
             files=files,
             force_multipart=True,
         )
 
-        return self._model_class.from_response(
-            response
-        )  # UNUSED type: ignore[attr-defined, no-any-return]
+        return self._model_class.from_response(response)
 
     def attachments(self, journal_id: str) -> JournalAttachmentsService:
         """Return journal attachments service."""
@@ -113,6 +116,9 @@ async def upload(
     ) -> Journal | ModelCollection[Journal]:  # noqa: WPS110
         """Upload journal file.
 
+        The journal file holds JSON Lines. Unless the caller declares a content type of its
+        own, the file is sent as ``application/x-ndjson``, which the endpoint accepts.
+
         Args:
             journal_id: Journal ID.
             file: journal file.
@@ -121,24 +127,22 @@ async def upload(
             Journal | ModelCollection[Journal]: The uploaded resource as a single Journal
             instance, or a ModelCollection[Journal] when the response contains multiple records.
         """
-        files = {}
+        files: dict[str, FileTypes] = {}
 
         if file:
-            files[self._upload_file_key] = file  # UNUSED type: ignore[attr-defined]
-            files[self._upload_data_key] = journal_id  # UNUSED type: ignore
+            files[self._upload_file_key] = declare_content_type(file, APPLICATION_X_NDJSON)
+            files[self._upload_data_key] = (None, journal_id)
 
         path = join_url_path(self.path, journal_id, "upload")
 
-        response = await self.http_client.request(  # UNUSED type: ignore[attr-defined]
+        response = await self.http_client.request(
             "post",
-            path,  # UNUSED type: ignore[attr-defined]
+            path,
             files=files,
             force_multipart=True,
         )
 
-        return self._model_class.from_response(
-            response
-        )  # UNUSED type: ignore[attr-defined, no-any-return]
+        return self._model_class.from_response(response)
 
     def attachments(self, journal_id: str) -> AsyncJournalAttachmentsService:
         """Return journal attachments service."""
 
@@ -1,7 +1,95 @@
+import asyncio
 import pathlib
+import time
 
 import pytest
 
+from mpt_api_client.exceptions import MPTError
+
+# A journal accepts DELETE only in these states; the API answers 400 for any other.
+DELETABLE_STATUSES = frozenset(("Draft", "Validated", "Error", "Enquiring"))
+# Uploading charges moves a journal through Validating for a second or so before it
+# settles. A journal that has been accepted and completed never becomes deletable.
+TRANSIENT_STATUSES = frozenset(("Validating", "Generating"))
+# A test that deletes the journal itself leaves nothing for the fixture to clean up.
+ALREADY_GONE_STATUSES = frozenset(("Deleted",))
+SETTLE_TIMEOUT_SECONDS = 10
+POLL_INTERVAL_SECONDS = 0.5
+
+
+def _report_teardown(message):
+    print(f"TEARDOWN - {message}")  # noqa: WPS421
+
+
+def _settled_status(journals_service, journal_id):
+    deadline = time.monotonic() + SETTLE_TIMEOUT_SECONDS
+    status = journals_service.get(journal_id).status
+    while status in TRANSIENT_STATUSES and time.monotonic() < deadline:
+        time.sleep(POLL_INTERVAL_SECONDS)
+        status = journals_service.get(journal_id).status
+
+    return status
+
+
+def _delete_journal(journals_service, journal_id):
+    try:
+        status = _settled_status(journals_service, journal_id)
+    except MPTError as error:
+        _report_teardown(f"could not read billing journal {journal_id}: {error}")
+        return
+
+    if status in ALREADY_GONE_STATUSES:
+        return
+
+    if status not in DELETABLE_STATUSES:
+        _report_teardown(f"billing journal {journal_id} not deletable in status {status}")
+        return
+
+    try:
+        journals_service.delete(journal_id)
+    except MPTError as error:
+        _report_teardown(f"could not delete billing journal {journal_id}: {error}")
+
+
+async def _async_settled_status(journals_service, journal_id):
+    deadline = time.monotonic() + SETTLE_TIMEOUT_SECONDS
+    journal = await journals_service.get(journal_id)
+    while journal.status in TRANSIENT_STATUSES and time.monotonic() < deadline:
+        await asyncio.sleep(POLL_INTERVAL_SECONDS)
+        journal = await journals_service.get(journal_id)
+
+    return journal.status
+
+
+async def _async_delete_journal(journals_service, journal_id):
+    try:
+        status = await _async_settled_status(journals_service, journal_id)
+    except MPTError as error:
+        _report_teardown(f"could not read billing journal {journal_id}: {error}")
+        return
+
+    if status in ALREADY_GONE_STATUSES:
+        return
+
+    if status not in DELETABLE_STATUSES:
+        _report_teardown(f"billing journal {journal_id} not deletable in status {status}")
+        return
+
+    try:
+        await journals_service.delete(journal_id)
+    except MPTError as error:
+        _report_teardown(f"could not delete billing journal {journal_id}: {error}")
+
+
+@pytest.fixture
+def delete_billing_journal():
+    return _delete_journal
+
+
+@pytest.fixture
+def async_delete_billing_journal():
+    return _async_delete_journal
+
 
 @pytest.fixture
 def billing_journal_fd():
 
@@ -7,7 +7,9 @@
 
 
 @pytest.fixture
-async def created_billing_journal(async_mpt_vendor, billing_journal_factory):
+async def created_billing_journal(
+    async_mpt_vendor, billing_journal_factory, async_delete_billing_journal
+):
     new_billing_journal_request_data = billing_journal_factory(
         name="E2E Created Billing Journal",
     )
@@ -18,10 +20,9 @@ async def created_billing_journal(async_mpt_vendor, billing_journal_factory):
 
     yield created_billing_journal
 
-    try:
-        await async_mpt_vendor.billing.journals.delete(created_billing_journal.id)
-    except MPTAPIError as error:
-        print(f"TEARDOWN - Unable to delete billing journal: {error.title}")  # noqa: WPS421
+    await async_delete_billing_journal(
+        async_mpt_vendor.billing.journals, created_billing_journal.id
+    )
 
 
 @pytest.fixture
 
@@ -7,7 +7,7 @@
 
 
 @pytest.fixture
-def created_billing_journal(mpt_vendor, billing_journal_factory):
+def created_billing_journal(mpt_vendor, billing_journal_factory, delete_billing_journal):
     new_billing_journal_request_data = billing_journal_factory(
         name="E2E Created Billing Journal",
     )
@@ -16,10 +16,7 @@ def created_billing_journal(mpt_vendor, billing_journal_factory):
 
     yield created_billing_journal
 
-    try:
-        mpt_vendor.billing.journals.delete(created_billing_journal.id)
-    except MPTAPIError as error:
-        print(f"TEARDOWN - Unable to delete billing journal: {error.title}")  # noqa: WPS421
+    delete_billing_journal(mpt_vendor.billing.journals, created_billing_journal.id)
 
 
 @pytest.fixture
 
@@ -0,0 +1,84 @@
+import io
+import os
+
+import pytest
+
+from mpt_api_client.http.file_utils import declare_content_type
+
+NDJSON = "application/x-ndjson"
+CSV = "text/csv"
+
+
+@pytest.mark.parametrize(
+    ("file_part", "expected"),
+    [
+        pytest.param(
+            b'{"id": 1}\n',
+            ("upload", b'{"id": 1}\n', NDJSON),
+            id="bytes without a name",
+        ),
+        pytest.param(
+            ("journal.jsonl", b'{"id": 1}\n'),
+            ("journal.jsonl", b'{"id": 1}\n', NDJSON),
+            id="filename and content",
+        ),
+        pytest.param(
+            ("journal.jsonl", b'{"id": 1}\n', None),
+            ("journal.jsonl", b'{"id": 1}\n', NDJSON),
+            id="content type left unset",
+        ),
+        pytest.param(
+            ("journal.jsonl", b'{"id": 1}\n', CSV),
+            ("journal.jsonl", b'{"id": 1}\n', CSV),
+            id="content type declared by the caller",
+        ),
+        pytest.param(
+            ("journal.jsonl", b'{"id": 1}\n', ""),
+            ("journal.jsonl", b'{"id": 1}\n', NDJSON),
+            id="blank content type treated as absent",
+        ),
+        pytest.param(
+            ("journal.jsonl", b'{"id": 1}\n', None, {"X-Trace": "1"}),
+            ("journal.jsonl", b'{"id": 1}\n', NDJSON, {"X-Trace": "1"}),
+            id="extra headers preserved",
+        ),
+        pytest.param(
+            (None, b'{"id": 1}\n'),
+            (None, b'{"id": 1}\n', NDJSON),
+            id="form field without a filename",
+        ),
+    ],
+)
+def test_declare_content_type(file_part, expected):
+    result = declare_content_type(file_part, NDJSON)
+
+    assert result == expected
+
+
+def test_filename_taken_from_file_object(tmp_path):
+    file_path = tmp_path / "journal.jsonl"
+    file_path.write_bytes(b'{"id": 1}\n')
+
+    with file_path.open("rb") as file_obj:
+        result = declare_content_type(file_obj, NDJSON)
+
+        assert result == ("journal.jsonl", file_obj, NDJSON)
+
+
+def test_filename_falls_back_without_name():
+    file_obj = io.BytesIO(b'{"id": 1}\n')
+
+    result = declare_content_type(file_obj, NDJSON)
+
+    assert result == ("upload", file_obj, NDJSON)
+
+
+def test_filename_from_file_descriptor(tmp_path):
+    file_path = tmp_path / "journal.jsonl"
+    file_path.write_bytes(b'{"id": 1}\n')
+    descriptor = os.open(file_path, os.O_RDONLY)
+
+    with os.fdopen(descriptor, "rb") as file_obj:
+        result = declare_content_type(file_obj, NDJSON)
+
+        assert result == (str(descriptor), file_obj, NDJSON)