Arduino IDE 1.8.13 Boards Manager download resume corrupts file

When installing a new Board Manager and canceling and resuming the download, the file gets corrupted.

I wanted to install the esp32 boards manager, download was toooo slow, I canceled and pressed install again - I thought I might get a faster download slot. At the end the resumed file failed the checksum test. I verified this with another file "littleBits Arduino AVR".

How to reproduce:
1. Install Arduino IDE 1.8.13 on Mac OS X
2. Add Board Manager URL https://dl.espressif.com/dl/package_esp32_index.json in Preferences
3. Install Tools / Board / Board Manager "esp32"
4. Wait for some MB to be downloaded, then cancel
5. Click on install again, it will resume the download. File size to download will show a different number
6. At the end you get a checksum error

Download just canceled, 500kb of 36800kb downloaded

Press install again and the target file size changes

At the end of the resumed download you get a checksum error message

Start with the Arduino IDE 1.x Board Manager download flow and reproduce the cancel-and-resume sequence using the esp32 package URL on Mac OS X. Compare the resumed file size with the expected download and verify that a completed download passes its checksum test.

Written by the indexing model from the issue text.