Blocks main thread on potentially slow file reads

[amake]

Picking a file potentially means downloading it from the internet in the case of e.g. an iCloud Drive remote file. This can be slow when the file is large or the network is slow. Unfortunately this plugin processes everything on the UI thread, so this hangs all of Flutter as well.

(Tested on iOS)

[amake]

Android is already using coroutines so it doesn't have this problem.