Mastodon's platform allows instance admins to customize post length, up to 5000 characters. The most popular instances including mastodon.social have a post limit of 500 characters. (Since launch in 2016.)
"Giving you more characters to express yourself" | Twitter Blog (September 26, 2017)
Twitter announces it's going to "try out" a doubled character limit (280) just five months after Mastodon's Big Week (when it was circulated around popular tech media.)
"New conversation settings, coming to a Tweet near you" | Twitter Blog (August 11, 2020)
As-is, Mastodon is not a file sharing service. Even though it allows people to attach images and videos to toots, there is no expectation of format or quality fidelity (e.g. max 8MB videos, webm/mp4 only, jpg/png only, large images are downsized to at most 1280^2 pixels). So even though you could encode a song in a video, it's far from sharing MP3 or FLAC files. You can't share ZIP archives. You can't send someone a PSD file.
I think it was a good decision to not allow audio uploads. Yes maybe it would've been fine. Or maybe the media would have branded Mastodon as a pirated music sharing service, and our project would start getting blocked from search results. Not a risk I am interested in taking at all when the most simple solution is to just include a link to soundcloud, instaud.io, vocaroo.com, dropbox, google cloud, your own server, or whatever else. Mastodon should not be a file sharing service!! It's a specialized problem domain with specialized solutions.
"Your Tweet, your voice" | Twitter Blog (June 17, 2020)
"Add audio uploads" - Tootsuite Version 2.9.1, June 22, 2019. (From Mastodon Changelog)
Mastodon's Delete and Redraft feature.
"If you could edit tweets" | Mastodon Blog (June 19, 2018)
Twitter seemed to be testing the exact same fucking function in September.
"image descriptions for screen readers," "Mastodon 2.0" (October 19, 2017)
@TwitterA11y. "Adding descriptions to images is a great way to include everyone in your conversation. These descriptions, aka alt-text, enable folks who use screen readers to interpret images in Tweets. Starting today, you no longer need a setting to add alt text and it's available on 📱 & 💻." Twitter, 27 May. 2020, 12:01 p.m., twitter.com/TwitterA11y/status/1265689579371323392.
My friend @Sentreh notes that the editable posts theory was a bug.