Checklist

• The bug is reproducible against the latest release or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

If you try to edit any fields of the form or submit the form unchanged, the image file that was uploaded earlier will be corrupted.

Here is my code:

from fastapi_storages import FileSystemStorage

storage = FileSystemStorage(path="static/images/")

class Subcategory(IntIdPkMixin, Base, Creatable, Updatable, SoftDeletable):
    name_en: Mapped[str] = mapped_column(String(50), nullable=False)
    name_ru: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)
    tag_en: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)
    tag_ru: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)
    image_url: Mapped[Optional[str]] = mapped_column(
        FileType(storage=storage), nullable=True
    )
    is_main: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
    slug: Mapped[Optional[str]] = mapped_column(String(100), nullable=True, unique=True)
    category_id: Mapped[int] = mapped_column(ForeignKey("categorys.id"))
    category: Mapped["Category"] = relationship(
        "Category", back_populates="subcategories"
    )
    products: Mapped[List["Product"]] = relationship(
        "Product", back_populates="subcategory"
    )

    def __str__(self):
        return self.name_ru

class SubcategoryAdmin(ModelView, model=Subcategory):
    name = "Подкатегории"
    name_plural = "Подкатегории"
    icon = "fa-solid fa-tags"
    category = "Товары"
    edit_template = "sqladmin/edit.html"

    column_list = [
        Subcategory.id,
        Subcategory.slug,
        Subcategory.name_en,
        Subcategory.name_ru,
        Subcategory.tag_en,
        Subcategory.tag_ru,
        Subcategory.is_main,
        "category.name_en",
        "category.name_ru",
    ]
    column_searchable_list = [Subcategory.name_en, Subcategory.name_ru]
    column_sortable_list = [
        Subcategory.id,
        Subcategory.slug,
        Subcategory.name_en,
        Subcategory.name_ru,
        Subcategory.tag_en,
        Subcategory.tag_ru,
        Subcategory.is_main,
        "category.name_en",
        "category.name_ru",
    ]
    can_create = True
    can_edit = True
    can_delete = True
    can_view_details = True

    form_columns = [
        "category",
        "name_en",
        "name_ru",
        "tag_en",
        "tag_ru",
        "image_url",
        "is_main",
    ]

    column_formatters = {
        "category.name_en": lambda m, a: m.category.name_en if m.category else "N/A",
        "category.name_ru": lambda m, a: m.category.name_ru if m.category else "N/A",
    }

    column_labels = {
        "id": "ID",
        "name_en": "Название подкатегории (англ.)",
        "name_ru": "Название подкатегории (рус.)",
        "tag_en": "Тег (англ.)",
        "tag_ru": "Тег (рус.)",
        "image_url": "URL изображения",
        "is_main": "Главная",
        "slug": "Слаг",
        "category_id": "ID Категории",
        "created_at": "Дата создания",
        "updated_at": "Дата обновления",
        "category.name_en": "Подкатегория (англ.)",
        "category.name_ru": "Подкатегория (рус.)",
        "products": "Продукты",
        "category": "Категория",
    }

    async def on_model_change(self, data, model, is_created, request):

        form = await request.form()

        for key, value in form.items():
            app_logger.info(
                f"Field name: {key}, Field type: {type(value)}, Field value: {value}"
            )

        if "image_url" in form:
            image = form["image_url"]

            if image.filename == "":
                data.pop("image_url", None)
                app_logger.info("Image field drop")

        model.slug = generate_slug(model.name_en)
        app_logger.info(data)
        async with Session(expire_on_commit=False) as session:
            subcategory_service = SubcategoryService(session)
            await subcategory_service.set_main_subcategory(model.id)

Steps to reproduce the bug

1.Go to the edit page
2. Save the edits without uploading a new image.
4. If you check the image that was there earlier, it will be damaged.

Expected behavior

it is expected to receive modified data in other fields, without damaging the image

Actual behavior

No response

Debugging material

I did a little research. I found that when submitting the form, if you do not embed a new image, there is file data in the form, but they are empty.
web_1 | INFO 2024-08-12 07:05:52.875 request id: None - core.admin:on_model_change - Field name: image_url, Field type: <class 'starlette.datastructures.UploadFile'>, Field value: UploadFile(filename='', size=0, headers=Headers({'content-disposition': 'form-data; name="image_url"; filename=""', 'content-type': 'application/octet-stream'}))

By adding a simple check and removing this field from the data, I avoid the problem of file corruption:
` async def on_model_change(self, data, model, is_created, request):

    form = await request.form()

    for key, value in form.items():
        app_logger.info(
            f"Field name: {key}, Field type: {type(value)}, Field value: {value}"
        )

    if "image_url" in form:
        image = form["image_url"]

        if image.filename == "":
            data.pop("image_url", None)
            app_logger.info("Image field drop")`

Environment

ubuntu 23.04, python 3.10, sqladmin[full] latest, fastapi 0.111, sqlalchemy[asyncio]~=2.0.30

Additional context

No response