Thunderbird sometimes doesn't download attachments completely, ending up with corrupt or wrong files.

By default, Thunderbird downloads attachments in chunks via IMAP and it doesn't download all of the chunks. You need to change the settings to force it to download attachments in a large chunk.

1) [Windows] Open Thunderbird, go to Tools > Options, and click Config Editor....

[macOS] Open Thunderbird, go to Preferences, select the General tab, and click Config Editor....

2) Click I accept the risk! (Required only when you open Config Editor for the first time)

3) Type mail.server.default.fetch_by_chunks in the Filter and double click on the item that shows up to make it false.

4) Close Config Editor.