@sotolf Sure, but they don't have to be uploaded to the server for you to write the alt text. Just show the image on the website/in the app, and upload after pushing publish…? No?
@forteller @sotolf It's the Web 2.0 way, media uploads and statuses are separate in the Mastodon API. This would have been useful e.g. if uploading a file type that cannot be displayed by the browser without a server-side conversion (I don't know if such are supported here).
I think a client application could easily simulate what you need, allowing all editing before posting the images and then uploading everything almost at once. I don't know which ones do that.