🎨 feat: Unified file upload UX — single attach button

Phase 3 of the unified file experience. When defaultFileInteraction is
configured (not 'legacy'), the attach file menu is replaced with a single
click-to-upload button that accepts all file types. Files are uploaded
without a tool_resource and provisioned lazily at chat time.

- AttachFileMenu: render single button in unified mode, legacy dropdown
  when defaultFileInteraction is 'legacy'
- validateFiles: accept union of all supported MIME types (provider +
  text + ocr + stt) in unified mode
- useFileHandling: already supports undefined tool_resource, no changes needed

client/src/components/Chat/Input/Files/AttachFileMenu.tsx

@@ -114,6 +114,10 @@ const AttachFileMenu = ({
     ephemeralAgent,
   );
 
+  const isUnifiedMode =
+    endpointFileConfig?.defaultFileInteraction != null &&
+    endpointFileConfig.defaultFileInteraction !== 'legacy';
+
   const handleUploadClick = useCallback(
     (fileType?: FileUploadType) => {
       if (!inputRef.current) {
@@ -139,11 +143,16 @@ const AttachFileMenu = ({
         inputRef.current.accept = '';
       }
       inputRef.current.click();
-      inputRef.current.accept = '';
     },
     [endpointFileConfig?.supportedMimeTypes],
   );
 
+  /** Unified mode: single click triggers file upload with no tool_resource */
+  const handleUnifiedUpload = () => {
+    setToolResource(undefined);
+    handleUploadClick();
+  };
+
   const dropdownItems = useMemo(() => {
     const setToolResource = (value: EToolResources | undefined) => {
       toolResourceRef.current = value;
@@ -305,6 +314,47 @@ const AttachFileMenu = ({
     }
   };
 
+  if (isUnifiedMode) {
+    return (
+      <>
+        <FileUpload
+          ref={inputRef}
+          handleFileChange={(e) => {
+            handleFileChange(e, toolResource);
+          }}
+        >
+          <TooltipAnchor
+            render={
+              <button
+                type="button"
+                disabled={isUploadDisabled}
+                id="attach-file-button"
+                aria-label={localize('com_sidepanel_attach_files')}
+                onClick={handleUnifiedUpload}
+                className="flex size-9 items-center justify-center rounded-full p-1 hover:bg-surface-hover focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary focus-visible:ring-opacity-50"
+              >
+                <div className="flex w-full items-center justify-center gap-2">
+                  <AttachmentIcon />
+                </div>
+              </button>
+            }
+            id="attach-file-button"
+            description={localize('com_sidepanel_attach_files')}
+            disabled={isUploadDisabled}
+          />
+        </FileUpload>
+        <SharePointPickerDialog
+          isOpen={isSharePointDialogOpen}
+          onOpenChange={setIsSharePointDialogOpen}
+          onFilesSelected={handleSharePointFilesSelected}
+          isDownloading={isProcessing}
+          downloadProgress={downloadProgress}
+          maxSelectionCount={endpointFileConfig?.fileLimit}
+        />
+      </>
+    );
+  }
+
   return (
     <>
       <FileUpload

client/src/utils/files.ts

@@ -273,8 +273,13 @@ export const validateFiles = ({
     }
 
     let mimeTypesToCheck = supportedMimeTypes;
-    if (toolResource === EToolResources.context) {
+    const isUnifiedMode =
+      !toolResource &&
+      endpointFileConfig.defaultFileInteraction != null &&
+      endpointFileConfig.defaultFileInteraction !== 'legacy';
+    if (toolResource === EToolResources.context || isUnifiedMode) {
       mimeTypesToCheck = [
+        ...(supportedMimeTypes || []),
         ...(fileConfig?.text?.supportedMimeTypes || []),
         ...(fileConfig?.ocr?.supportedMimeTypes || []),
         ...(fileConfig?.stt?.supportedMimeTypes || []),