Also I made a reacton component for testing this code based on solara.FileDrop. If this PR gets merged, I can send a PR with something similar to solara repository.

import threading
from typing import Callable, TypedDict, BinaryIO, cast

import reacton
import solara

from ipyvuetify.extra.file_input import FileInput


class FileInfo(TypedDict):
    name: str
    size: int
    lastModified: int
    type: str
    file_obj: BinaryIO
    data: bytes | None


@reacton.component
def Upload(on_upload: Callable[[list[FileInfo]], None] = lambda _: None, lazy: bool = False, **kwargs):
    file_info, set_file_info = reacton.use_state(None)
    wired_files, set_wired_files = reacton.use_state(cast(list[FileInfo] | None, None))

    file_input = FileInput.element(
        outlined=True,
        counter=True,
        multiple=True,
        show_size=True,
        label="Files",
        placeholder="Select files...",
        chips=True,
        color="purple",
        on_v_model=set_file_info,
        disabled=bool(file_info),
        loading=bool(file_info),
        prepend_icon="mdi-upload",
        attributes={"accept": "image/*"},
        **kwargs
    )

    def wire_files():
        if not file_info:
            return

        real = cast(FileInput, reacton.get_widget(file_input))

        # workaround for @observe being cleared
        real.version += 1
        real.reset_stats()

        set_wired_files(cast(list[FileInfo], real.get_files()))

    reacton.use_effect(wire_files, [file_info])

    def handle_file(cancel: threading.Event):
        if not wired_files:
            return
        if on_upload:
            for wired_file in wired_files:
                wired_file["data"] = None if lazy else wired_file["file_obj"].read()
            on_upload(wired_files)
        set_file_info(None)

    result: solara.Result = solara.hooks.use_thread(handle_file, [wired_files])
    if result.error:
        raise result.error

    return file_input


if __name__ == "__main__":

    @reacton.component
    def Page():
        Upload(on_upload=print)

Yeah, this is great! I didn't know you could override a slot by adding it again. I also overlooked there was a slot for progress.

I agree with @maartenbreddels that we should not show warnings for the changed properties. In the notebook these will show up for the end-user. That end-user may not be the author and may not know how to fix it.

Looks like only show_progress is not backward compatible. It would be great if we could make that backward compatible as-well. Then this feature would have no breaking changes.

Note: I will force push to this branch to remove the erroneous commit that was temporarily on master.

I see there are lint-errors. This can be prevented by using pre-commit. I've just updated the readme with these instructions:

$ pip install -e ".[dev]"
$ pre-commit install

This will check and enforce code style when committing.

To update the existing code you can run:

$ pre-commit run --all-files

I've removed warnings and fixed code style issues.
Also, I returned the sync tag to file_info and used get_files() to set v_model value on file_info change. Now it is possible to remove the effect from the component above and use just on_v_model=set_wired_files.

I have a question: why this workaround is needed for solara.FileDrop? How observe can be cleared?

real.version += 1
real.reset_stats()

UPD: show_progress is still not backward compatible because it was True by default, and I'm not sure about doing the same for loading.

I added a computed property as a default loading state. If neither loading nor show_progress are set, the progress bar will appear automatically, but it is also easy to control the state manually.

I also thought about transforming v_model values into objects, that use pythonic key names (last_modified instead of lastModified) and maybe even allow attribute access with dot notation (namedtuple?). It should be easier to do now, before this PR is merged. What do you think?

UPD: I've almost finished adding drag & drop 😃

Sorry for the late response, I missed the notifications for the changes on this PR.

I added a computed property as a default loading state. If neither loading nor show_progress are set, the progress bar will appear automatically, but it is also easy to control the state manually.

Wouldn't it be better to leave out loading from the API, since it's controlled by the component internally?

The drag & drop functionality is really nice!, but it shouldn't be part of this PR. Are you able to move those commits to a new PR (that can be based on this PR)?