Expose SetFilterer to set filterer function of FileButton's FilePicker

[bqback]

Describe the feature

FileButton only allows setting filenames and extensions, which merely offers highlighting items that match the criteria. This offers no actual value beyond highlighting and (at least on Windows) fundamentally differs from how the "Extensions" field works in a regular (OS level) file picker.

FilePIcker supports providing a filter function that does what "Extensions" should do, but apparently it's not possible to bind to FilePIcker because it doesn't have a WidgetValue, and the internal filepicker of a FileButton isn't exposed anywhere at all.

Relevant code

No response

[kkoreilly]

Thank you for reporting this. We will expose the filter function on FileButton and make Extensions filter items instead of just highlighting them.