WinSCP uses content type according to Windows registry.
You probably do not have wepb registered.
Either add an entry manually:
Mime Types in the Windows registry
Or configure an override in WinSCP using Interface\MimeTypes raw configuration setting:
https://winscp.net/eng/docs/rawconfig#mimetypes

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Similar question:
Upload js files to S3 with wrong content-type

Hi,
I am trying to upload a large amount of webp images to Backblaze B2. And apparently this changes the format from image/webp to binary/octet-stream what causes a small problem if an users wants to view the images. It will be downloaded to the users computer if he views the image.

If I use the upload function from Backblaze there is no change, and a user can view the image in their browser.

With other image formats I do not have this problem.

Update: uploaded with Cyberduck, and also no problems,