Wrong attachment size showed at arrival

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

Thunderbird for Android

App version

12.0b6

Where did you get the app from?

F-Droid

Android version

14

Device model

Samsung Galaxy S21 5G

Steps to reproduce

1. Send an attachment from desktop Thunderbird (I am using https://ftp.mozilla.org/pub/thunderbird-mobile/android/nightly/latest-main/thunderbird-13.0a1.apk).
2. Check the size of the file (it's around 12MB)
3. Receive the email with the attachment on Android.
4. Check what's displayed as a size when you first read the e-mail.
5. It shows 17.5 MB (see the screenshot)

For a safety aware person this might mean a lot - someone tampered the file while transfering.

A note: If you visit the email the second time, it shows a correct number.

Expected behavior

it should show the correct size of the attachment

Actual behavior

it shows a wrong size

Logs

No response

Start by reproducing the attachment-size display on Android 14 with Thunderbird for Android 12.0b6, using the listed desktop and nightly app setup. Compare the size shown on first read with the size shown on a subsequent visit; done means the initial display matches the correct attachment size.