there’s no Linux client that supports lossless

Do you mean ALAC files? I downloaded a sample at https://getsamplefiles.com/sample-audio-files/alac and played it in VLC under Linux. There are several players that can play libavcodec FFmpeg.

On my Mint 22 Wilma setup using Xfce, using Firefox 139.0 (64-bit) mint-001 - 1.0, when I click the “upload image” button here in lemmy, I see small thumbnails for .jpg and .png files, but not for .webp.

Maybe the .webp format is too new?

Ctrl++ doesn’t work in my “File Upload” dialog box to make the thumbnails bigger.

Edit: I always rename files I download to give them descriptive file names, starting with the most important word first.