Use transcoded image when original format not supported

[gnprice]

If someone uploads a file in the TIFF image format, I believe we'll attempt to show a preview of the image, and if the user taps it we'll attempt to show it in the lightbox. But I don't expect the image will successfully appear.

The Zulip server is gaining a feature that will enable us to show these: it'll transcode such files to more widely-supported formats. This uses the thumbnailing system but the image is much larger than "thumbnail" size. So it would be nice to use that feature.

This is a low priority because I believe TIFF files are not commonly used. (The main motivation for introducing the feature was for HEIC/HEIF files, which are much more common. But HEIF support is provided by the system both on iOS and on Android 11+, and Flutter uses the system's facilities for image formats, so this app already supports HEIF just fine.)

References

• #api design
• lightbox: Present fallbacks for unsupported image types (e.g., HEIC) zulip#32587
• Serve high-resolution transcoded versions of HEIC/TIFF images zulip#32930
• An analogous issue for videos (but there aren't currently plans to have the Zulip server transcode videos, which would be much more expensive):
  lightbox: Offer to open browser if video player doesn't support a video format #1208