Downloading an image on iOS shows a .dat file in the share sheet and never saves

On iOS, opening an image attachment and tapping the download button opens the share sheet, but the file shows up as a .dat file instead of an image. There is no option to save it to Photos, nothing useful happens, and the download button keeps spinning.

Expected: the share sheet shows the image (with Save Image) and the button stops loading once the sheet closes.

Saving images used to work (shipped April 2025), so this looks like a regression. Seen in 1.77 on iOS.