Different favicons depending on if folder or files are open

[jancborchardt]

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Is your feature request related to a problem? Please describe.
Originally proposed by our colleague Katrin:

When I have a window open or bookmark a folder or file, I can't see what it is, cause they have the same icon.
It makes it hard to smoothly toggle between windows and tasks, as I am often have to recall whether I opened the folder or file.
Not sure if I am the only one struggling with this but an icon to indicate "file" would be very helpful. 🙏

Describe the solution you'd like
It would be nice indeed since I can confirm I had the same issue recently as well.
This could possibly be solved by using the mimetype icon for the relevant file as favicon if any file is open in direct links, shares or when they are open in Files. Since there is a blue background on the favicon, we should use the monochrome white version of whichever mimetype icon we use.

Describe alternatives you've considered
Just using a generic "File" favicon if there is any kind of file shown might already be good, but it's too simple and doesn't work for video files, images, etc.

For images and PDFs we could even consider using the first page preview as favicon, although that might me too much and not quickly identifiable as "Nextcloud" anymore.

@marcoambrosini @sorbaugh