Unicode filenames of attachments replaced by underscore when opened

This only happens when opening the attachment file with non-ascii name, e.g., by view-attach or view-mailcap.

I tested on mutt 2.0.6 with an empty file named 中文名称.txt (I also tried something like ɑαѷ✓) and the following .mailcap:

text/plain; vim %s

If I send this file to my email, and open this file with view-mailcap command, vim shows the file name as ____________.txt, with 12 underscores, which has the same bytes as the Chinese basename of the file in utf-8 encoding.

The same happens when open pdf files in pdf viewers, documents in LibreOffice, etc.

The filename is correct anywhere else, such as what's shown in the attachments screen, or when saving the file to local storage.