fix(frontend): keep the given file name on mobile downloads

FileManager typed every download text/plain. Android's MediaStore appends
the MIME type's extension whenever the name's own extension maps elsewhere,
so a subscriber saving a WireGuard config got peer.conf.txt, which the
WireGuard app refuses; .json, .yaml and .log downloads were renamed the same
way. Desktop browsers honour the download name, which is why only phones
saw it. application/octet-stream carries no extension of its own, so the
name the panel chose is kept.

frontend/src/test/file-download.test.tsx

@@ -0,0 +1,35 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { FileManager } from '@/utils';
+
+const { createObjectURL, revokeObjectURL } = URL;
+
+afterEach(() => {
+  URL.createObjectURL = createObjectURL;
+  URL.revokeObjectURL = revokeObjectURL;
+  vi.restoreAllMocks();
+});
+
+describe('FileManager.downloadTextFile', () => {
+  // Android's MediaStore appends the blob type's own extension when the name's
+  // extension maps elsewhere, so a text/plain peer.conf lands as peer.conf.txt.
+  it('hands the browser an untyped blob so a mobile save keeps the given name', () => {
+    const blobs: Blob[] = [];
+    URL.createObjectURL = vi.fn((blob: Blob) => {
+      blobs.push(blob);
+      return 'blob:download';
+    });
+    URL.revokeObjectURL = vi.fn();
+    let savedAs = '';
+    vi.spyOn(HTMLAnchorElement.prototype, 'click').mockImplementation(function (
+      this: HTMLAnchorElement,
+    ) {
+      savedAs = this.download;
+    });
+
+    FileManager.downloadTextFile('[Interface]\n', 'alice.conf');
+
+    expect(savedAs).toBe('alice.conf');
+    expect(blobs.map((blob) => blob.type)).toEqual(['application/octet-stream']);
+  });
+});