Handle download in progress gracefully

[devzsolt]

If a file is not "on disk" in Nextcloud but it needs to be downloaded from Sia network, the app always returns a meaningless textfile regardless of the mime type of the real file.
This behavior causes broken operations in Nextcloud (gallery view, PDF preview, save multiple files as a zip, etc.)
It's a major UX problem that needs to be addressed.
I see this simplification in the code here:

Sia-Nextcloud/lib/Storage/Sia.php

Line 353 in 39ed3ee

return fopen('data://text/plain,' . $path . ' is currently being downloaded from Sia.', $mode);

but I'm not sure how could Nextcloud handle it in an async way, showing a load indicator instead of providing a fake file instantly.