Modified https://github.com/XeroAPI/xero-python-oauth2-starter for downloading expense claims receipts attachments.

Note that files are only available via Files API. Attachements API is misleading as per Xero dev support email below:

Date: Mon, 30 Jun 2025 08:37:26 +0000 (GMT)
From: Xero Support <[email protected]>
To: "Ivan"
Subject: Xero Support - Xero API Support: receipts attachments -
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The attachments endpoint was created after the deprecation of the expenses
endpoint and so expenses were not included in the scope of the GET or PUT/POST
requests for attachments.

I have done some testing with API Explorer and an old test account that still
has expenses and I was able to use the Files API to get files from Xero and
these included files attached to expense claims.