I run Obsidian 0.11.5 on Linux Mint 20.1/Cinnamon. Renaming to .tiff makes Obsidian recognize the image as valid file type (it shows up in the attachments folder). If I click it in the files pane, an external image viewing tool opens and shows it. But is is not displayed as image in preview mode, as the OP showed.
Then again, most browsers don’t display .tiff natively, so maybe this is an Electron limitation?