diff --git a/components/files/__tests__/file-preview-modal-history.test.tsx b/components/files/__tests__/file-preview-modal-history.test.tsx
index b29dcbbfb..fd7c0d356 100644
--- a/components/files/__tests__/file-preview-modal-history.test.tsx
+++ b/components/files/__tests__/file-preview-modal-history.test.tsx
@@ -204,3 +204,82 @@ describe('FilePreviewModal history lifecycle (modeled queued traversal)', () =>
     expect(screen.queryByRole('dialog')).toBeNull();
   });
 });
+
+// A forwarded certified-mail (PEC) message: postacert.eml wraps the original
+// as msg.eml, which used to be download-only inside the .eml preview.
+const INNER_EML = [
+  'From: Sender <sender@example.com>',
+  'To: me@example.com',
+  'Subject: Inner original',
+  'Date: Tue, 27 Oct 2026 20:36:28 +0100',
+  'Content-Type: text/plain; charset=UTF-8',
+  '',
+  'the original message',
+].join('\r\n');
+const OUTER_EML = [
+  'From: Forwarder <fwd@example.com>',
+  'To: me@example.com',
+  'Subject: FWD: wrapper',
+  'Date: Wed, 28 Oct 2026 05:50:39 +0100',
+  'MIME-Version: 1.0',
+  'Content-Type: multipart/mixed; boundary="b1"',
+  '',
+  '--b1',
+  'Content-Type: text/plain; charset=UTF-8',
+  '',
+  'see attached',
+  '--b1',
+  'Content-Type: message/rfc822; name="msg.eml"',
+  'Content-Disposition: attachment; filename="msg.eml"',
+  '',
+  INNER_EML,
+  '--b1--',
+  '',
+].join('\r\n');
+
+describe('FilePreviewModal attachment of a previewed .eml', () => {
+  async function openOuter() {
+    const getContent = vi.fn().mockResolvedValue({
+      blob: new Blob([OUTER_EML], { type: 'message/rfc822' }), contentType: 'message/rfc822',
+    });
+    const view = openPreview(false, getContent, 'postacert.eml');
+    await advance(0);
+    await vi.waitFor(() => expect(screen.getByText('FWD: wrapper')).toBeInTheDocument());
+    fireEvent.click(screen.getByText('msg.eml'));
+    await advance(0);
+    await vi.waitFor(() => expect(screen.getByText('Inner original')).toBeInTheDocument());
+    return view;
+  }
+
+  it('opens in a preview of its own in place of the outer one, with a Back entry', async () => {
+    await openOuter();
+    await advance(0);
+    expect(screen.getAllByRole('dialog')).toHaveLength(1);
+    expect(screen.getByRole('dialog')).toHaveAccessibleName('msg.eml');
+    expect(push).toHaveBeenCalledTimes(2);
+  });
+
+  it('Escape closes the inner preview only', async () => {
+    const { onClose } = await openOuter();
+    fireEvent.keyDown(window, { key: 'Escape' });
+    expect(screen.getByRole('dialog')).toHaveAccessibleName('postacert.eml');
+    expect(screen.getByText('FWD: wrapper')).toBeInTheDocument();
+    expect(onClose).not.toHaveBeenCalled();
+    fireEvent.keyDown(window, { key: 'Escape' });
+    expect(onClose).toHaveBeenCalledOnce();
+  });
+
+  it('Browser Back closes the inner preview, the next Back the outer one', async () => {
+    const { onClose } = await openOuter();
+    await advance(0);
+    window.history.back();
+    await advance(10);
+    expect(screen.getByRole('dialog')).toHaveAccessibleName('postacert.eml');
+    expect(onClose).not.toHaveBeenCalled();
+    expect(window.history.state.__closeOnBack).toBeTruthy();
+    window.history.back();
+    await advance(10);
+    expect(onClose).toHaveBeenCalledOnce();
+    expect(window.history.state).toEqual(originalState);
+  });
+});
diff --git a/components/files/eml-preview.tsx b/components/files/eml-preview.tsx
index 27e73ddcf..0ff73b06e 100644
--- a/components/files/eml-preview.tsx
+++ b/components/files/eml-preview.tsx
@@ -5,6 +5,7 @@ import { Paperclip, Download } from "@/components/icons";
 import { emailIframeCsp, sanitizeEmailBodyForIframe } from "@/lib/email-sanitization";
 import { getEffectiveTimeZone } from "@/lib/timezone";
 import { stripBidiControls } from "@/lib/bidi";
+import { isFilePreviewable } from "@/lib/file-preview";
 import { useSettingsStore } from "@/stores/settings-store";
 import { useNoThirdPartyTraffic } from "@/hooks/use-third-party-mode";
 
@@ -17,9 +18,11 @@ export type ParsedEml = {
   date?: string;
   html?: string;
   text?: string;
-  attachments?: Array<{ filename?: string; mimeType?: string; content?: ArrayBuffer | Uint8Array }>;
+  attachments?: Array<EmlAttachment>;
 };
 
+export type EmlAttachment = { filename?: string; mimeType?: string; content?: ArrayBuffer | Uint8Array };
+
 function formatAddress(a?: { name?: string; address?: string }): string {
   if (!a) return "";
   if (a.name && a.address) return `${a.name} <${a.address}>`;
@@ -41,12 +44,19 @@ function escapeHtml(s: string): string {
 // scripts, no same-origin), so a script-bearing .eml can never execute in our
 // origin. Remote content is blocked unless the user allows it everywhere:
 // there is no per-message "load" here. Parsing happens in the caller
-// (FilePreviewModal); this is pure presentation.
-export function EmlPreview({ message }: { message: ParsedEml }) {
+// (FilePreviewModal); this is pure presentation. An attachment the preview
+// can show - often another message: a certified-mail (PEC) envelope wraps the
+// original, and a forwarded PEC wraps that envelope - opens through
+// onOpenAttachment, with download beside it.
+export function EmlPreview({ message, onOpenAttachment }: {
+  message: ParsedEml;
+  onOpenAttachment?: (attachment: EmlAttachment) => void;
+}) {
   const t = useTranslations("email_viewer");
   // Always blocked with no third-party traffic (#903).
   const remoteBlockedByOrg = useNoThirdPartyTraffic();
   const blockRemote = useSettingsStore((state) => state.externalContentPolicy) !== "allow" || remoteBlockedByOrg;
+  const tFiles = useTranslations("files");
 
   // dir="auto" on a wrapper we control lets the first strong character pick
   // the direction, matching the main viewer.
@@ -60,7 +70,7 @@ export function EmlPreview({ message }: { message: ParsedEml }) {
     ? `<!DOCTYPE html><html><head><meta charset="utf-8"><meta http-equiv="Content-Security-Policy" content="${emailIframeCsp(blockRemote)}"><meta name="referrer" content="no-referrer"></head><body>${body}</body></html>`
     : "";
 
-  const downloadAttachment = (att: NonNullable<ParsedEml["attachments"]>[number]) => {
+  const downloadAttachment = (att: EmlAttachment) => {
     if (!att.content) return;
     // content is a real ArrayBuffer/Uint8Array at runtime; cast for the strict
     // BlobPart lib type (Uint8Array<ArrayBufferLike> vs ArrayBuffer).
@@ -103,19 +113,47 @@ export function EmlPreview({ message }: { message: ParsedEml }) {
         <div className="mt-4 border-t border-border pt-3">
           <div className="text-xs font-medium text-muted-foreground mb-2">{t("attachments")}</div>
           <div className="flex flex-wrap gap-2">
-            {message.attachments.map((att, i) => (
-              <button
-                key={i}
-                type="button"
-                onClick={() => downloadAttachment(att)}
-                title={t("download")}
-                className="flex items-center gap-2 px-3 py-1.5 rounded-md text-sm bg-muted text-foreground hover:bg-muted/70"
-              >
-                <Paperclip className="w-3 h-3 flex-shrink-0" />
-                <span className="max-w-[200px] truncate">{att.filename ? stripBidiControls(att.filename) : "attachment"}</span>
-                <Download className="w-3 h-3 flex-shrink-0 text-muted-foreground" />
-              </button>
-            ))}
+            {message.attachments.map((att, i) => {
+              const name = att.filename ? stripBidiControls(att.filename) : "attachment";
+              const opens = !!onOpenAttachment && !!att.content && isFilePreviewable(att.filename, att.mimeType);
+              if (!opens) {
+                return (
+                  <button
+                    key={i}
+                    type="button"
+                    onClick={() => downloadAttachment(att)}
+                    title={t("download")}
+                    className="flex items-center gap-2 px-3 py-1.5 rounded-md text-sm bg-muted text-foreground hover:bg-muted/70"
+                  >
+                    <Paperclip className="w-3 h-3 flex-shrink-0" />
+                    <span className="max-w-[200px] truncate">{name}</span>
+                    <Download className="w-3 h-3 flex-shrink-0 text-muted-foreground" />
+                  </button>
+                );
+              }
+              return (
+                <div key={i} className="flex items-center rounded-md text-sm bg-muted text-foreground">
+                  <button
+                    type="button"
+                    onClick={() => onOpenAttachment(att)}
+                    title={tFiles("preview")}
+                    className="flex items-center gap-2 ps-3 pe-2 py-1.5 rounded-s-md hover:bg-muted/70"
+                  >
+                    <Paperclip className="w-3 h-3 flex-shrink-0" />
+                    <span className="max-w-[200px] truncate">{name}</span>
+                  </button>
+                  <button
+                    type="button"
+                    onClick={() => downloadAttachment(att)}
+                    title={t("download")}
+                    aria-label={`${t("download")} ${name}`}
+                    className="ps-1 pe-3 py-1.5 rounded-e-md hover:bg-muted/70"
+                  >
+                    <Download className="w-3 h-3 text-muted-foreground" />
+                  </button>
+                </div>
+              );
+            })}
           </div>
         </div>
       )}
diff --git a/components/files/file-preview-modal.tsx b/components/files/file-preview-modal.tsx
index a1136b0d6..b1bbc10c1 100644
--- a/components/files/file-preview-modal.tsx
+++ b/components/files/file-preview-modal.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useEffect } from "react";
+import { useState, useEffect, useMemo } from "react";
 import { useTranslations } from "next-intl";
 import { ExternalLink } from "@/components/icons";
 import { Spinner } from "@/components/ui/spinner";
@@ -9,9 +9,10 @@ import { IconButton } from "@/components/ui/icon-button";
 import { ZoomableImage } from "@/components/files/zoomable-image";
 import { getFilePreviewKind, imageBlobUrl, isMimeTypeSafeForInlinePreview, previewBlobType } from "@/lib/file-preview";
 import dynamic from "next/dynamic";
-import { EmlPreview, type ParsedEml } from "@/components/files/eml-preview";
+import { EmlPreview, type EmlAttachment, type ParsedEml } from "@/components/files/eml-preview";
 import { FilePreviewPluginSection } from "@/components/files/file-preview-plugin-section";
 import { canOpenUrlInNewWindow } from "@/lib/standalone-display";
+import { stripBidiControls } from "@/lib/bidi";
 import { emailIframeCsp, sanitizeEmailBodyForIframe } from "@/lib/email-sanitization";
 import { useSettingsStore } from "@/stores/settings-store";
 import { useCloseOnBack } from "@/hooks/use-close-on-back";
@@ -165,6 +166,10 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent, fi
   // Always blocked with no third-party traffic (#903).
   const remoteBlockedByOrg = useNoThirdPartyTraffic();
   const blockRemote = useSettingsStore((state) => state.externalContentPolicy) !== "allow" || remoteBlockedByOrg;
+  // An attachment of the previewed .eml, shown in a preview of its own in
+  // place of this one. It holds a Back entry of its own, so Escape and Back
+  // close it first and come back here.
+  const [nestedPreview, setNestedPreview] = useState<{ name: string; blob: Blob } | null>(null);
 
   // Decide whether to render the PDF in a plain <iframe> (desktop) or with the
   // pdf.js canvas viewer (mobile). navigator.pdfViewerEnabled is the standard
@@ -266,6 +271,47 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent, fi
     };
   }, [getFileContent, name, blockRemote]);
 
+  const openEmlAttachment = (att: EmlAttachment) => {
+    if (!att.content) return;
+    setNestedPreview({
+      name: att.filename ? stripBidiControls(att.filename) : "attachment",
+      // content is a real ArrayBuffer/Uint8Array at runtime; cast for the
+      // strict BlobPart lib type, as EmlPreview's download does.
+      blob: new Blob([att.content as BlobPart], { type: att.mimeType || "application/octet-stream" }),
+    });
+  };
+  // Stable per attachment: the loader effect re-runs when it changes.
+  const nestedContent = useMemo(
+    () => nestedPreview
+      ? async () => ({ blob: nestedPreview.blob, contentType: nestedPreview.blob.type })
+      : null,
+    [nestedPreview],
+  );
+  const downloadNested = () => {
+    if (!nestedPreview) return;
+    const url = URL.createObjectURL(nestedPreview.blob);
+    const a = document.createElement("a");
+    a.href = url;
+    a.download = nestedPreview.name;
+    document.body.appendChild(a);
+    a.click();
+    document.body.removeChild(a);
+    URL.revokeObjectURL(url);
+  };
+
+  // In place of this frame, not on top of it: one frame at a time takes
+  // Escape and the arrows. This one keeps its state and its Back entry.
+  if (nestedPreview && nestedContent) {
+    return (
+      <FilePreviewModal
+        name={nestedPreview.name}
+        onClose={() => setNestedPreview(null)}
+        onDownload={downloadNested}
+        getFileContent={nestedContent}
+      />
+    );
+  }
+
   return (
     <PreviewShell
       name={name}
@@ -306,7 +352,7 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent, fi
         )}
 
         {!loading && !error && fileType === "eml" && emlContent && (
-          <EmlPreview message={emlContent} />
+          <EmlPreview message={emlContent} onOpenAttachment={openEmlAttachment} />
         )}
 
         {!loading && !error && fileType === "image" && objectUrl && (