No Chapters in Audiobooks Uploaded to Files (M4a or M4b)

User reported that when they upload an M4A custom file to Pocket Casts, the chapters get removed. Reported in 5931028-zen.

### Expected behavior
When you upload audiobooks in M4a format to Files, you should be able to navigate the chapters.

### Actual behavior
The app merges the chapters in M4a files into a single file.

### Steps to reproduce the behavior
1. Open an app that has the audiobook (M4A file).
2. Choose to share that file.
3. Save to Pocket Casts.

@joashrajin was able to reproduce this. Slack discussion: p1675760418992739-slack-C02A333D8LQ

##### User details
App Version: 7.30
Device: iPhone13,2
OS: 16.0

Start by reproducing the issue in the Pocket Casts iOS Files flow with an audiobook in M4A or M4B format, then trace where the shared file is imported and chapters are handled. Done means uploaded audiobooks retain navigable chapters instead of being merged into one file.

Written by the indexing model from the issue text.