Manual:$wgTiffThumbnailType/de-formal

Details
Array of file extension and MIME type to convert TIFF images to when sending them for inline display in web pages. By default, this is false, meaning this conversion does not occur.

Browsers don't support TIFF inline generally, so images need to be converted to an alternate format. Note scaling should work with ImageMagick, but may not with GD scaling.