A WebXDC app could be used to manually add an alttext to a file when sending it to a chat. The user would need to manually attach the app to the chat, though, then attach the image in the app, a slower and kludgier UI.
Having a UI option to pin a minimized draft-message-editing app to the bottom of a chat might make app functionality much more useful for this sort of purpose. It could be used for custom emoji-pickers, other special characters, stickerpacks, shared-secret encryption, file-format conversion, unit conversion, and other output-centric uses, for which sharing the WebXDC zip file is not really needed.
There have been people on the forum who assemble and distribute groups of files and designate these groups as stickerpacks, as I recall. An ordinary WebXDC app could be used to collaboratively collect and caption images for this purpose, and would encourage alttext use.