public FileManagerSettingsToolbar SettingsToolbar { get; }

Public ReadOnly Property SettingsToolbar As FileManagerSettingsToolbar

<dx:ASPxFileManager ID="FileManager" ClientInstanceName="FileManager" runat="server" />
    <SettingsToolbar>
        <Items>
            <dx:FileManagerToolbarRefreshButton BeginGroup="false" />
            <dx:FileManagerToolbarCustomButton Text="Thumbnails View" CommandName="ChangeView-Thumbnails" 
            GroupName="ViewMode">
                <Image IconID="grid_cards_16x16" />
            </dx:FileManagerToolbarCustomButton>
            ...
        </Items>
    </SettingsToolbar>
</dx:ASPxFileManager>

ASPxFileManager fm = new ASPxFileManager();
...
FileManagerToolbarRefreshButton refresh = new FileManagerToolbarRefreshButton();
refresh.BeginGroup = false;

FileManagerToolbarCustomButton customButton = new FileManagerToolbarCustomButton();
customButton.Text = "Thumbnails View";
customButton.CommandName = "ChangeView-Thumbnails";
customButton.GroupName = "ViewMode";
customButton.Image.IconID = "grid_cards_16x16";

fm.SettingsToolbar.Items.Add(refresh);
fm.SettingsToolbar.Items.Add(customButton);