Attach files from compose box, with file picker

[gnprice]

This Zulip feature: https://zulip.com/help/share-and-upload-files

Not to be confused with an image picker, #56. This feature can be represented by a "paperclip" icon, rather than "picture". The resulting UI allows selecting from a wider range of files, and has a different set of affordances less focused on images.

After the user selects a file, they should have the opportunity to add text before sending the message. (We got that right in the RN app longer ago than for images: zulip/zulip-mobile#4590.) Like for the image picker, this probably means doing the same thing as in the RN app and the web app: we immediately insert a placeholder into the message text, then start the upload, and when the file is uploaded and we have a URL for it we fill in the placeholder.

For handling the platform side of this from Flutter:

• It looks like the popular solution is the file_picker package.

  • Way too few open issues (20 of 1094) — they must be using stale-bot.
    • Well, they are — 208 closed issues look like its work.
    • But that’s a small fraction of the issues, and none of the top-voted ones. So it's actually a pretty mild case of the stale-bot.
  • They handled the Android scoped-storage transition reasonably promptly, in 2020: [Feature] Support scoped storage on Android miguelpruivo/flutter_file_picker#234
  • Generally seems fairly well maintained. (I reached that conclusion 2023-02-01 from looking closer, and no new concerns jump out at me on a quicker look now.)

• There's also a Flutter first-party plugin file_selector…

  • … but not yet for Android: [file_selector] Add Android support flutter/flutter#110098
    • So it won't yet work for us.
  • ​​The last platform they added before that was iOS. I guess they found more demand for this functionality in desktop and web apps — which makes sense.