The GP comment:
> FTP clients are native to every popular OS, from Android to Windows (only exceptions I know are Win Mobile and iOS)
To rephrase: only 1/3 of mobile OSes support FTP.
Not working on iOS is like saying "Oh, this road doesn't work with German made cars. That's not a big deal, is it?"
https://panic.com/transmit-ios/
Searching the App Store I also see some apps for SMB, but I don't know whether they have document provider extensions.
FWIW, I'm not convinced that "web-based" is a better alternative for read/write file access, assuming you mean file manager webapps. No OS can integrate those into the native file picker, so you can't avoid the inefficiency of manually uploading files after changing them. WebDAV works pretty well though, if that counts...
Yeah, nope. Dead out of the gate. Thanks for playing.