`FileUpload`'s `save_file` function returns path incorrectly on Windows with backslash separator (due to `os.path.join`)

Title says it all.

When using `from fastapi_admin.file_upload import FileUpload` and it returns the save path (endpoint to fetch the file) using prefix and filename, it joins the path using `os.path.join`, which is OS-dependent. This causes the filename to contain `\` separated path on Windows.

Example:
```python
upload_face = FileUpload(
uploads_dir="./static/uploads",
prefix="/faces", # StaticFiles is mounted here
filename_generator=random_filename, # Ignore this
)
```

It just causes the backslash escape character sequence to mess up path when fetching images sometimes.