whole-message atttachments not displayed

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App version

5.807

Where did you get the app from?

Google Play

Android version

8.1.0

Device model

Redmi Go

Steps to reproduce

Receive mail where the message's content-type is (say) application/pdf attachment, to be precise
MIME-Version: 1.0
Content-Type: application/pdf; name="Ticket_2.pdf"
Content-Disposition: attachment; filename="Ticket_2.pdf"
Content-Transfer-Encoding: base64

Expected behavior

Display the message body as an attachment button with its size, and allow me to download/view by pressing the button.

Actual behavior

Display the attachment button with size label of "0 B". Pressing the button creates and views an empty file in the download directory.

Logs

I don' t have time right now to go though the log creation and retrieval, and I imagine it's not the kind of bug where a log is needed. But if the log is needed, comment, and I'll go about getting one.

Reproduce the issue in Thunderbird for Android 5.807 using a message whose entire content is an application/pdf attachment with the MIME headers shown. Trace the attachment handling and download path, then verify that the message shows the correct size and that downloading or viewing produces the original attachment rather than an empty file.