file-picker-dialog
kate uses kde5 file-picker-dialog
GtkFileChooser GtkFileChooserDialog
You should install oxygen icon pack:
sudo apt-get install oxygen-icon-theme*
and if the left side panel (file list..) is missing:
sudo apt-get install kdelibs-bin kdelibs5-data kdelibs5-plugins
"Kate uses QT file dialog not KDE's"
The only workaround I've found is to use Konqueror to browse the filesystem, and then use the right-click context menu to open the file with Kate.
Note that installing Kubuntu-desktop and/or Plasma instead of Unity does NOT fix the problem with Kate.
The file chooser and save as dialogues are related to the graphical toolkit upon which an application is built, and in no way related to default file manager. Since most programs use gtk toolkit under Ubuntu, I see no possible way of altering those dialogues. There's possibility of switching to Qt-based desktops, but at least in my experience under razorqt that didn't do the trick. Therefore my answer to this question is altering file dialogues is impossible. There's other, more experienced users that me, hence perhaps there might be other answer, so my answer isn't conclusive
GTK File Chooser Sucks
KGtk is the hackish way to use KDE dialogs in GTK+ applications. It was first released in mid-2006 and made available on KDE-Apps.org as upstream and also via AUR package.
While this sounds promising, note that this work intention is to give a uniform look for Qt and GTK+ applications. Most likely, it does not address the slow issues which have been debated by the users.
KGtk seems to be the only way to replace GTK+ dialog and it must be applied per application. There is no known alternative to actually replace the dialogs for system-wide case. Else, look for workarounds to make GTK+ file chooser load faster (This would be another
https://wiki.archlinux.org/index.php/Uniform_look_for_Qt_and_GTK_applications#QGtkStyle
gtkfilechooser.ini
[Filechooser Settings] LocationMode=path-bar ShowHidden=false ShowSizeColumn=true GeometryX=0 GeometryY=0 GeometryWidth=732 GeometryHeight=540 SortColumn=name SortOrder=ascending StartupMode=recent
In GTK+ 2.x, the file chooser saves its state and configuration options in a gtk-2.0/gtkfilechooser.ini file under the directory that g_get_user_config_dir() returns. (On Unix, this usually resolves to $HOME/username/.config/gtk-2.0/gtkfilechooser.ini.) While some of the available options can be changed directly through the file chooser's user interface, a couple are only editable by hand or by third-party tools (such as gnome-tweak-tool). This section describes the available options.
This is a sample of the contents of a gtkfilechooser.ini file. Note that all the following options go under a [Filechooser Settings] heading.
[Filechooser Settings] LocationMode=filename-entry ShowHidden=false ExpandFolders=true GeometryX=570 GeometryY=273 GeometryWidth=780 GeometryHeight=585 ShowSizeColumn=true SortColumn=name SortOrder=ascending StartupMode=recent
LocationMode key
The LocationMode key controls whether the file chooser shows just a path bar, or a visible entry for the filename as well, for the benefit of typing-oriented users. The possible string values for these modes are path-bar and filename-entry, respectively.
ShowHidden key
The ShowHidden key controls whether the file chooser shows hidden files or not. The value can be be true or false.
ShowSizeColumn key
The ShowSize key controls whether the file chooser shows a column with file sizes. The value can be be true or false.
Geometry keys
The four keys GeometryX, GeometryY, GeometryWidth, GeometryHeight save the position and dimensions of the GtkFileChooserDialog's window.
SortColumn key
The SortColumn key can be one of the strings name, modified, or size. It controls which of the columns in the file chooser is used for sorting the list of files.
SortOrder key
The SortOrder key can be one of the strings ascending or descending.
StartupMode key
The StartupMode key controls whether the file chooser starts up showing the list of recently-used files, or the contents of the current working directory. Respectively, the values can be recent or cwd.
Kate new tab placement
Although reddit sucks, here is the link: https://www.reddit.com/r/kde/comments/bforo2/kate_new_tab_placement/