Several file formats (notably PDF, DjVu, SVG, and TIFF) show in-browser previews, using the links under the image on its description page, in different file formats (JPG and PNG) than the original file, but the interface does not warn the user of this anywhere. All it says is:
"Size of this preview: 463 × 599 pixels. Other resolutions: [...]"
For these file types, the message should specify, e.g., "These preview images will be rendered in JPG", much as several projects are already doing through site-specific JS (see SVGs on Commons: https://commons.wikimedia.org/wiki/File:Svg_example4.svg "This image rendered as PNG in other widths: 200px, 500px, 1000px, 2000px."). This is also important usability-wise because the preview file formats are typically more user-friendly than the original files. We should be making users aware that when they are looking at a large file in unfamiliar TIFF format that they are able to simply download JPGs from the preview links.