Enhance FileChooser to allow better app-level handling of file rejections

[amcclain]

The underlying react-dropzone that powers this component can be configured to reject files based on type and size, both of which are exposed as options in our API within FileChooserProps.

However the handling around rejected files is very limited. The model will set an observable lastRejectedCount property when a rejection occurs, but this does not appear to be cleared or otherwise incremented. This means that if you attempt to add a single file and its rejected, you will see the count go to 1, but if you try another (single) file the count will remain at 1 and no observable change occurs (this I would say is a straight-up bug, or at least useless enough to be effectively broken).

There is some code in the Hoist component that reads this value and looks like it should show a message, but I'm not seeing this work at all with our in-app usage.

The underlying API provides much more info on rejections in the form of a FileRejectionEvent with nested errors including messages that (presumably) help to explain what happened. If that were properly observable, or if the app could directly register a custom handler, an app could decide how it wanted to relay info to a user about why a file failed to "take" when dragged over the chooser or selected from the OS file browser. Right now it's just this silent failure, with no apparent workaround.

[amcclain]

While we have this component open for surgery, let's also discuss if we want to make a breaking change to move its primary configuration to the model vs. component props.

[amcclain]

In case we don't get to take this ticket on in the near future, here was a workaround I managed to implement - this function is run in a reaction to fileChooserModel.lastRejectedCount - note the need to reach in and reset that count to zero, as well as (continued) inability to explain precisely why we didn't accept it:

onFileRejected(rejected) {
        if (rejected > 0) {
            dangerToast(
                `Unable to accept file for upload - it may exceed the size limit of ${Math.round(
                    this.maxFileSize / 1000000
                )}MB or be of an unsupported type.`
            );
            runInAction(() => {
                this.fileChooserModel.lastRejectedCount = 0;
            });
        }
    }