From 4aebae3f24ababe8d421c6271919cf715f253e3a Mon Sep 17 00:00:00 2001
From: Matthew Batema <github@lv1fetch.quest>
Date: Fri, 11 Apr 2025 10:04:49 -0700
Subject: [PATCH] Fix downcast from Starlette UploadFile to FastAPI UploadFile

---
 fastapi/datastructures.py             | 12 +++++
 fastapi/routing.py                    | 16 ++++++-
 tests/test_request_uploadfile_type.py | 63 +++++++++++++++++++++++++++
 3 files changed, 90 insertions(+), 1 deletion(-)
 create mode 100644 tests/test_request_uploadfile_type.py

diff --git a/fastapi/datastructures.py b/fastapi/datastructures.py
index cf8406b0f..47412ab1f 100644
--- a/fastapi/datastructures.py
+++ b/fastapi/datastructures.py
@@ -5,6 +5,7 @@ from typing import (
     Dict,
     Iterable,
     Optional,
+    Self,
     Type,
     TypeVar,
     cast,
@@ -72,6 +73,17 @@ class UploadFile(StarletteUploadFile):
         Optional[str], Doc("The content type of the request, from the headers.")
     ]
 
+    @classmethod
+    def from_starlette(
+        cls: type[Self], starlette_uploadfile: StarletteUploadFile
+    ) -> Self:
+        return cls(
+            file=starlette_uploadfile.file,
+            size=starlette_uploadfile.size,
+            filename=starlette_uploadfile.filename,
+            headers=starlette_uploadfile.headers,
+        )
+
     async def write(
         self,
         data: Annotated[
diff --git a/fastapi/routing.py b/fastapi/routing.py
index 54c75a027..546e105fc 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -31,7 +31,7 @@ from fastapi._compat import (
     _normalize_errors,
     lenient_issubclass,
 )
-from fastapi.datastructures import Default, DefaultPlaceholder
+from fastapi.datastructures import Default, DefaultPlaceholder, UploadFile
 from fastapi.dependencies.models import Dependant
 from fastapi.dependencies.utils import (
     _should_embed_body_fields,
@@ -60,6 +60,7 @@ from fastapi.utils import (
 from pydantic import BaseModel
 from starlette import routing
 from starlette.concurrency import run_in_threadpool
+from starlette.datastructures import UploadFile as StarletteUploadFile
 from starlette.exceptions import HTTPException
 from starlette.requests import Request
 from starlette.responses import JSONResponse, Response
@@ -209,6 +210,19 @@ async def run_endpoint_function(
     # facilitate profiling endpoints, since inner functions are harder to profile.
     assert dependant.call is not None, "dependant.call must be a function"
 
+    # Convert all Starlette UploadFiles to FastAPI UploadFiles
+    for key, value in values.items():
+        if isinstance(value, StarletteUploadFile) and not isinstance(value, UploadFile):
+            values[key] = UploadFile.from_starlette(value)
+        elif isinstance(value, list):
+            values[key] = [
+                UploadFile.from_starlette(item)
+                if isinstance(item, StarletteUploadFile)
+                and not isinstance(item, UploadFile)
+                else item
+                for item in value
+            ]
+
     if is_coroutine:
         return await dependant.call(**values)
     else:
diff --git a/tests/test_request_uploadfile_type.py b/tests/test_request_uploadfile_type.py
new file mode 100644
index 000000000..adeca0ed6
--- /dev/null
+++ b/tests/test_request_uploadfile_type.py
@@ -0,0 +1,63 @@
+import io
+from typing import Any
+
+from fastapi import FastAPI, File, UploadFile
+from fastapi.testclient import TestClient
+from starlette.datastructures import UploadFile as StarletteUploadFile
+
+app = FastAPI()
+
+
+@app.post("/uploadfile")
+async def uploadfile(uploadfile: UploadFile = File(...)) -> dict[str, Any]:
+    return {
+        "filename": uploadfile.filename,
+        "is_fastapi_uploadfile": isinstance(uploadfile, UploadFile),
+        "is_starlette_uploadfile": isinstance(uploadfile, StarletteUploadFile),
+        "class": f"{uploadfile.__class__.__module__}.{uploadfile.__class__.__name__}",
+    }
+
+
+@app.post("/uploadfiles")
+async def uploadfiles(
+    uploadfiles: list[UploadFile] = File(...),
+) -> list[dict[str, Any]]:
+    return [
+        {
+            "filename": uploadfile.filename,
+            "is_fastapi_uploadfile": isinstance(uploadfile, UploadFile),
+            "is_starlette_uploadfile": isinstance(uploadfile, StarletteUploadFile),
+            "class": f"{uploadfile.__class__.__module__}.{uploadfile.__class__.__name__}",
+        }
+        for uploadfile in uploadfiles
+    ]
+
+
+def test_uploadfile_type() -> None:
+    client = TestClient(app)
+    files = {"uploadfile": ("example.txt", io.BytesIO(b"test content"), "text/plain")}
+    response = client.post("/uploadfile/", files=files)
+    data = response.json()
+
+    assert data["filename"] == "example.txt"
+    assert data["is_fastapi_uploadfile"] is True
+    assert data["is_starlette_uploadfile"] is True
+    assert data["class"].startswith("fastapi.")
+
+
+def test_uploadfiles_type() -> None:
+    client = TestClient(app)
+    files = [
+        ("uploadfiles", ("example.txt", io.BytesIO(b"test content"), "text/plain"))
+    ]
+    response = client.post("/uploadfiles/", files=files)
+    files_data = response.json()
+
+    assert len(files_data) == 1
+
+    data = files_data[0]
+
+    assert data["filename"] == "example.txt"
+    assert data["is_fastapi_uploadfile"] is True
+    assert data["is_starlette_uploadfile"] is True
+    assert data["class"].startswith("fastapi.")