Document the file_icon function

docs/dev/reference/twig/functions/file_icon.md

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+---
+title: file_icon - Twig Function
+linkTitle: file_icon
+description: Helper function render an icon based on a file's type in the back end.
+tags: [Twig]
+---
+
+{{< version "5.7" >}}
+
+With this function you can generate a back end icon based on a file's type. This is used for the back end in the
+template of the download(s) content element for example.
+
+The function takes a `FilesystemItem` as its mandatory argument and also an `alt` attribute as well as an
+`HtmlAttribute` instance as optional arguments.
+
+```twig
+{{ file_icon(download.file, 'Download', attrs().addClass('file-icon')) }}
+```
+
+## Arguments
+
+* `item`: The `FilesystemItem` to generate the file icon for.
+* `alt`: The string for the `alt` attribute of the `<img>`.
+* `attributes`: An `HtmlAttributes` instance for additional HTML attributes on the `<img>`.