Here's an idea: instead of showing a field called "Filter" below the "Name field", we turn it into a real search field that becomes visible from a toolbar button or something. GNOME and macOS have a search feature in their Open/Save dialogs and I find it really handy, whereas the current filter functionality is not very useful IMO and is only sometimes available (sometimes it turns into a file type chooser combobox).
This looks excellent as well!
@ngraham I believe on Windows, some applications have a hybrid. An editable combo box that allows you to select an extension or type in a custom one. Windows implements this rather oddly in some cases (for example, the appending of an extension seems to have odd behavior for some applications). Tldr; I agree with you 100%, as long as the combobox functionality itself is kept.