Background

UploadFile is just a wrapper around SpooledTemporaryFile, which can be accessed as UploadFile.file.

SpooledTemporaryFile() [...] function operates exactly as TemporaryFile() does

And documentation about TemporaryFile says:

Return a file-like object that can be used as a temporary storage area. [..] It will be destroyed as soon as it is closed (including an implicit close when the object is garbage collected). Under Unix, the directory entry for the file is either not created at all or is removed immediately after the file is created. Other platforms do not support this; your code should not rely on a temporary file created using this function having or not having a visible name in the file system.

async def endpoint

You should use the following async methods of UploadFile: write, read, seek and close. They are executed in a thread pool and awaited asynchronously.

For async writing files to disk you can use aiofiles. Example:

@app.post("/")
async def post_endpoint(in_file: UploadFile=File(...)):
    # ...
    async with aiofiles.open(out_file_path, 'wb') as out_file:
        content = await in_file.read()  # async read
        await out_file.write(content)  # async write

    return {"Result": "OK"}

Or in the chunked manner, so as not to load the entire file into memory:

@app.post("/")
async def post_endpoint(in_file: UploadFile=File(...)):
    # ...
    async with aiofiles.open(out_file_path, 'wb') as out_file:
        while content := await in_file.read(1024):  # async read chunk
            await out_file.write(content)  # async write chunk

    return {"Result": "OK"}

def endpoint

Also, I would like to cite several useful utility functions from this topic (all credits @dmontagu) using shutil.copyfileobj with internal UploadFile.file. This functions can be invoked from def endpoints:

import shutil
from pathlib import Path
from tempfile import NamedTemporaryFile
from typing import Callable

from fastapi import UploadFile


def save_upload_file(upload_file: UploadFile, destination: Path) -> None:
    try:
        with destination.open("wb") as buffer:
            shutil.copyfileobj(upload_file.file, buffer)
    finally:
        upload_file.file.close()


def save_upload_file_tmp(upload_file: UploadFile) -> Path:
    try:
        suffix = Path(upload_file.filename).suffix
        with NamedTemporaryFile(delete=False, suffix=suffix) as tmp:
            shutil.copyfileobj(upload_file.file, tmp)
            tmp_path = Path(tmp.name)
    finally:
        upload_file.file.close()
    return tmp_path


def handle_upload_file(
    upload_file: UploadFile, handler: Callable[[Path], None]
) -> None:
    tmp_path = save_upload_file_tmp(upload_file)
    try:
        handler(tmp_path)  # Do something with the saved temp file
    finally:
        tmp_path.unlink()  # Delete the temp file

Note: you'd want to use the above functions inside of def endpoints, not async def, since they make use of blocking APIs.