diff --git a/src/api/__tests__/live-stalwart-compose.test.ts b/src/api/__tests__/live-stalwart-compose.test.ts
index 5b2b922e..123f26e3 100644
--- a/src/api/__tests__/live-stalwart-compose.test.ts
+++ b/src/api/__tests__/live-stalwart-compose.test.ts
@@ -199,6 +199,51 @@ live('live Stalwart: composer and sending (0.16 and 1.0)', () => {
       expect(saved.attachments.map((a) => a.name)).toContain('from-files.txt');
     }, 60_000);
 
+    const text = (buf: ArrayBuffer) => new TextDecoder().decode(new Uint8Array(buf));
+    const nodeBlob = async (id: string) => {
+      const res = await singleton.request(
+        [['FileNode/get', { accountId: singleton.accountId, ids: [id] }, '0']],
+        [CAPABILITIES.CORE, CAPABILITIES.FILES],
+      );
+      return res.methodResponses[0][1].list[0] as { blobId: string; name: string };
+    };
+
+    // 0.16 made a FileNode from a part's blobId hold the whole raw message.
+    it("files a message's attachment part as the attachment's bytes, under a free name, and attaches it back", async () => {
+      const content = `part ${RUN}`;
+      const upload = await uploadBytes(new TextEncoder().encode(content), 'text/plain');
+      const draft = await saveDraftVersion({
+        from: [{ email: USER }], to: [{ email: USER }], subject: `part ${tag}`, textBody: 'see attached',
+        attachments: [{ blobId: upload.blobId, name: 'part.txt', type: 'text/plain', disposition: 'attachment' }],
+      }, drafts);
+      createdEmails.push(draft.id);
+      const [full] = await getFullEmails([draft.id]);
+      const part = full.attachments!.find((a) => a.name === 'part.txt')!;
+      expect(part.blobId).toBeTruthy();
+
+      const name = `${tag}-part.txt`;
+      const first = await saveAttachmentToFiles({ blobId: part.blobId!, name, type: 'text/plain' }, null);
+      createdNodes.push(first.id);
+      const stored = await nodeBlob(first.id);
+      expect(text(await singleton.fetchBlobArrayBuffer(stored.blobId, name, 'text/plain'))).toBe(content);
+
+      // The same name again: the server would refuse it (alreadyExists).
+      const second = await saveAttachmentToFiles({ blobId: part.blobId!, name, type: 'text/plain' }, null);
+      createdNodes.push(second.id);
+      expect(second.name).toBe(`${tag}-part (1).txt`);
+
+      // And back: the file attached to a new draft carries the same bytes.
+      const back = await attachmentFromFileNode({ ...stored, id: first.id, nodeType: 'file', type: 'text/plain' });
+      const reply = await saveDraftVersion({
+        from: [{ email: USER }], to: [{ email: USER }], subject: `back ${tag}`, textBody: 'see attached',
+        attachments: [{ blobId: back.blobId, name: 'back.txt', type: 'text/plain', disposition: 'attachment' }],
+      }, drafts);
+      createdEmails.push(reply.id);
+      const [again] = await getFullEmails([reply.id]);
+      const attached = again.attachments!.find((a) => a.name === 'back.txt')!;
+      expect(text(await singleton.fetchBlobArrayBuffer(attached.blobId!, 'back.txt', 'text/plain'))).toBe(content);
+    }, 60_000);
+
     it('copies blobs with Blob/copy only when the blob capability is in `using`', async () => {
       const upload = await uploadBytes(new TextEncoder().encode(`copy ${RUN}`), 'text/plain');
       const call = ['Blob/copy', { fromAccountId: singleton.accountId, accountId: singleton.accountId, blobIds: [upload.blobId] }, '0'];
diff --git a/src/api/files.ts b/src/api/files.ts
index d618f601..8907d2e2 100644
--- a/src/api/files.ts
+++ b/src/api/files.ts
@@ -426,6 +426,17 @@ function usesDraftFileNodeCreate(accountId: string): boolean {
   return !!cap && typeof cap === 'object' && 'supportedNodeTypes' in cap;
 }
 
+/**
+ * Whether a FileNode made from a message part's blobId holds that part.
+ * Stalwart 1.0.0 stores the part; 0.16 stores the whole raw message the part
+ * is in under the part's name (seen on 0.16.27), so there the part's bytes
+ * have to be uploaded as a blob of their own first. Told apart as for the
+ * create shape: only 1.0.0 implements File Storage draft 18.
+ */
+export function fileNodeTakesPartBlobs(accountId: string = filesAccountId()): boolean {
+  return usesDraftFileNodeCreate(accountId);
+}
+
 /** One FileNode/set create in `accountId`, in the shape that server takes. */
 async function createFileNodeIn(
   accountId: string,
diff --git a/src/lib/__tests__/compose-files.test.ts b/src/lib/__tests__/compose-files.test.ts
index 07010c62..11c060ca 100644
--- a/src/lib/__tests__/compose-files.test.ts
+++ b/src/lib/__tests__/compose-files.test.ts
@@ -1,6 +1,6 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
 
-const { files, email } = vi.hoisted(() => ({
+const { files, email, blob, client } = vi.hoisted(() => ({
   files: {
     filesAccountId: vi.fn(() => 'me'),
     copyFileNode: vi.fn(async (node: { blobId: string }, parentId: string | null, name: string) => ({
@@ -8,16 +8,30 @@ const { files, email } = vi.hoisted(() => ({
     })),
     fileNodeKind: vi.fn((node: { nodeType?: string; blobId?: string | null }) =>
       node.nodeType ?? (node.blobId == null ? 'directory' : 'file')),
+    // Stalwart 1.0.0 unless a test says otherwise.
+    fileNodeTakesPartBlobs: vi.fn(() => true),
+    getAllFileNodes: vi.fn(async (): Promise<Array<Record<string, unknown>>> => []),
+    getFileNameRules: vi.fn(() => null),
   },
   email: {
     // As the real one: the ids as they are within one account.
     copyBlobs: vi.fn(async (from: string, ids: string[], to: string) =>
       Object.fromEntries(ids.map((id) => [id, from === to ? id : `copy-of-${id}`]))),
   },
+  blob: {
+    uploadBytes: vi.fn(async (_bytes: Uint8Array, type: string, accountId?: string) => ({
+      blobId: `uploaded-to-${accountId}`, size: 3, type,
+    })),
+  },
+  client: {
+    accountId: 'me',
+    fetchBlobArrayBuffer: vi.fn(async () => new Uint8Array([1, 2, 3]).buffer),
+  },
 }));
 vi.mock('../../api/files', () => files);
 vi.mock('../../api/email', () => email);
-vi.mock('../../api/jmap-client', () => ({ jmapClient: { accountId: 'me' } }));
+vi.mock('../../api/blob', () => blob);
+vi.mock('../../api/jmap-client', () => ({ jmapClient: client }));
 
 import type { FileNode } from '../../api/types';
 import {
@@ -92,4 +106,35 @@ describe('Save to Files', () => {
       expect.objectContaining({ blobId: 'copy-of-att-1' }), null, 'invoice.pdf',
     );
   });
+
+  // 0.16 filed the whole raw message under the attachment's name.
+  it("uploads the attachment's bytes into the Files account on Stalwart 0.16", async () => {
+    files.fileNodeTakesPartBlobs.mockReturnValue(false);
+    try {
+      await saveAttachmentToFiles({ blobId: 'att-1', name: 'invoice.pdf', type: 'application/pdf' }, 'd1', 'team');
+    } finally {
+      files.fileNodeTakesPartBlobs.mockReturnValue(true);
+    }
+    expect(client.fetchBlobArrayBuffer).toHaveBeenCalledWith('att-1', 'invoice.pdf', 'application/pdf', 'team');
+    expect(blob.uploadBytes).toHaveBeenCalledWith(new Uint8Array([1, 2, 3]), 'application/pdf', 'me');
+    expect(email.copyBlobs).not.toHaveBeenCalled();
+    expect(files.copyFileNode).toHaveBeenCalledWith(
+      expect.objectContaining({ blobId: 'uploaded-to-me' }), 'd1', 'invoice.pdf',
+    );
+  });
+
+  it('numbers the name when the folder holds one like it', async () => {
+    files.getAllFileNodes.mockResolvedValueOnce([
+      { id: 'x', name: 'invoice.pdf', parentId: 'd1' },
+      { id: 'y', name: 'invoice (1).pdf', parentId: 'd1' },
+      { id: 'z', name: 'other.pdf', parentId: null },
+      { id: 's', name: 'other (1).pdf', parentId: null, isShared: true },
+    ]);
+    await saveAttachmentToFiles({ blobId: 'att-1', name: 'invoice.pdf', type: 'application/pdf' }, 'd1');
+    expect(files.copyFileNode).toHaveBeenLastCalledWith(expect.anything(), 'd1', 'invoice (2).pdf');
+
+    files.getAllFileNodes.mockResolvedValueOnce([{ id: 'x', name: 'invoice.pdf', parentId: 'd1' }]);
+    await saveAttachmentToFiles({ blobId: 'att-1', name: 'invoice.pdf', type: 'application/pdf' }, null);
+    expect(files.copyFileNode).toHaveBeenLastCalledWith(expect.anything(), null, 'invoice.pdf');
+  });
 });
diff --git a/src/lib/compose-files.ts b/src/lib/compose-files.ts
index bd56ad95..52649d98 100644
--- a/src/lib/compose-files.ts
+++ b/src/lib/compose-files.ts
@@ -1,14 +1,21 @@
 // Mail and Files meet here: a file from Files attached to a message (#1179)
-// and an attachment filed in Files (webmail save-to-files-dialog). Neither
-// downloads or uploads anything: both refer to a blob the server already
-// has. Stalwart 0.16.27 and 1.0.0 take a FileNode's blob as an attachment
-// and an attachment's blob for a FileNode within one account; across
-// accounts the blob is copied first (Blob/copy), as the webmail does.
+// and an attachment filed in Files (webmail save-to-files-dialog). Both
+// refer to a blob the server already has where they can. Stalwart 0.16.27
+// and 1.0.0 take a FileNode's blob as an attachment, and 1.0.0 an
+// attachment's blob for a FileNode; across accounts the blob is copied first
+// (Blob/copy), as the webmail does. On 0.16 a FileNode made from an
+// attachment's blob holds the whole message, so there the attachment is
+// downloaded and uploaded again.
 
+import { uploadBytes } from '../api/blob';
 import { copyBlobs } from '../api/email';
-import { copyFileNode, fileNodeKind, filesAccountId } from '../api/files';
+import {
+  copyFileNode, fileNodeKind, fileNodeTakesPartBlobs, filesAccountId, getAllFileNodes, getFileNameRules,
+} from '../api/files';
 import { jmapClient } from '../api/jmap-client';
 import type { FileNode } from '../api/types';
+import { acceptedFileName } from './file-name-rules';
+import { getUniqueName } from './filenode-name';
 
 /** The Files trash the webmail keeps (lib/files-trash): hidden like any dot folder. */
 const TRASH_FOLDER_NAME = '.Trash';
@@ -73,7 +80,9 @@ export async function attachmentFromFileNode(
  * File an attachment in the user's Files, in `parentId` (null: My files).
  * `blobAccountId` is the account the message is in when it is not the
  * login's own: a shared or group mailbox's blob is copied into the Files
- * account first.
+ * account first. On Stalwart 0.16 the attachment's bytes are uploaded into
+ * the Files account instead (see fileNodeTakesPartBlobs). A name the folder
+ * has already gets a number.
  */
 export async function saveAttachmentToFiles(
   attachment: { blobId: string; name?: string | null; type?: string | null },
@@ -81,8 +90,23 @@ export async function saveAttachmentToFiles(
   blobAccountId?: string,
 ): Promise<FileNode> {
   const from = blobAccountId ?? jmapClient.accountId;
-  const blobId = (await copyBlobs(from, [attachment.blobId], filesAccountId()))[attachment.blobId];
-  const name = attachment.name || 'attachment';
-  const node: FileNode = { id: '', name, type: attachment.type || 'application/octet-stream', blobId, nodeType: 'file' };
+  const to = filesAccountId();
+  const type = attachment.type || 'application/octet-stream';
+  // A name the folder holds already is refused (alreadyExists): the copy
+  // gets " (1)", " (2)", ... as an upload does.
+  const taken = new Set(
+    (await getAllFileNodes())
+      .filter((n) => !n.isShared && (n.parentId ?? null) === parentId)
+      .map((n) => n.name),
+  );
+  const name = getUniqueName(acceptedFileName(attachment.name || 'attachment', getFileNameRules()), taken);
+  let blobId: string;
+  if (fileNodeTakesPartBlobs(to)) {
+    blobId = (await copyBlobs(from, [attachment.blobId], to))[attachment.blobId];
+  } else {
+    const bytes = await jmapClient.fetchBlobArrayBuffer(attachment.blobId, name, type, from);
+    blobId = (await uploadBytes(new Uint8Array(bytes), type, to)).blobId;
+  }
+  const node: FileNode = { id: '', name, type, blobId, nodeType: 'file' };
   return copyFileNode(node, parentId, name);
 }