support File Ext & MIME type validation for uploaded files

[Damon0820]

Sometimes we encounter scenarios where we need to add MIME type （https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/input/file#accept） validation for uploaded files. For example, in image uploads, we might restrict the allowed file types to “.png, .jpeg”. In that case, we would need to add MIME type validation consistently across all entry points, including selecting a system image in MediaToolbarButton, entering an image URL in MediaUrlDialogContent, as well as onDrop, onPaste, and so on.
Could you please help evaluate whether this feature can be added? Thanks!