New Post: How to provide search parameters to url input dialog?

Basically, You need to add a new Viewdefinition on the page :

<script type="text/javascript">

            if (!ViewDefinitions["You.Namespace.XsltFileSelectorDialog"]) {
                ViewDefinitions["You.Namespace.XsltFileSelectorDialog"] = new DialogViewDefinition({
                    isMutable: true,
                    handle: "Composite.Management.EmbeddableMediaSelectorDialog",
                    position: Dialog.MODAL,
                    url: Dialog.URL_TREEACTIONSELECTOR,
                    argument: {
                        label: "${string:Composite.Management:Website.FrontendFile.SelectDialog.Title}",
                        image: "${icon:media}",
                        selectionProperty: "ElementType",
                        selectionValue: null,
                        selectionResult: "Uri",
                        nodes: [
                            {
                                key: "LayoutFileElementProvider",
                                search: "Composite.C1Console.Elements.SearchToken,Composite|Keyword='\.css'"
                            }
                        ],
                        width: 480
                    }
                });
            }
        </script>

And than use:

<ui:fielddata>
    <ui:datainputdialog handle="You.Namespace.XsltFileSelectorDialog" name="datainputdialog1" value="1" type="integer"/>
</ui:fielddata>

But C1 does not provide filtering by extension. Default filter by keyword also check folder
So you can see in tree only somelike:
/test.css/
/test.css/style.css


Also you can write custom provider. instead 'LayoutFileElementProvider'