Open a TIFF image and view it in the browser, browsers cannot show TIFF natively, with its dimensions and a page selector for multi-page files.
The viewer decodes the TIFF with the UTIF library in the browser and draws the decoded pixels onto a canvas, reporting the image’s width and height. TIFF files can hold several images (pages) in one file, so when UTIF returns more than one image directory the viewer adds a page selector and decodes the chosen page on demand. Most common TIFF variants from scanners, cameras and image editors decode this way; some less common compression schemes are outside the decoder’s coverage, and those files produce an error message instead of a blank result. Nothing is uploaded: decoding happens entirely on the local machine.
Why can a TIFF not just be opened in a browser tab? Browsers do not include a TIFF decoder, so a .tif file normally downloads instead of displaying. This viewer decodes the TIFF data in the browser and draws it onto a canvas.
What happens with a multi-page TIFF? Faxes and scans often store several pages in one TIFF. When the decoder finds more than one page, a page selector appears so each page can be viewed in turn.
Which TIFF files might fail? Unusual compression schemes and very large files can exceed what the in-browser decoder handles. A failure message is shown rather than a blank page, and a desktop image viewer is the fallback for those files.