Files · Items: one icon map, file icon, file row and file grid

The crate had two maps from a file's name to its icon, in chat and in the
file tree, and they disagreed. One map now lives in primitives, over the
Lucide file-* icons, and chat, agent, the file tree and the directory
listing use it. FileIcon draws it for a file or a folder. FileItem is a
file's row, a ListItem with its size and when it changed. FileList is the
directory listing. FileGrid lays tiles in the order given, with a cursor
seeded from the owner's pick: arrows by tile and by row, Home, End, Enter
and a double press to open. The count of what fits a width has one home,
layout's columns_for, and fit builds on it.

src/chat/media.rs

@@ -10,7 +10,7 @@ use crate::{
     buttons::{ButtonVariant, IconButton},
     documents::source,
     forms::{Pick, Run},
-    primitives::{Icon, IconName, Image},
+    primitives::{Icon, IconName, Image, file_icon},
     theme::{ActiveTheme, AvatarSize, ContainerSize, IconSize, Radius, TextSize},
     typography::{Ellipsis, format::file_size},
 };
@@ -205,16 +205,6 @@ impl RenderOnce for ImageGrid {
     }
 }
 
-/// The icon a file's name suggests.
-pub(crate) fn file_icon(name: &str) -> IconName {
-    let extension = name.rsplit_once('.').map(|(_, ext)| ext.to_lowercase());
-    match extension.as_deref() {
-        Some("png" | "jpg" | "jpeg" | "gif" | "webp" | "svg" | "heic") => IconName::Image,
-        Some("pdf" | "md" | "txt" | "doc" | "docx" | "rtf" | "pages") => IconName::FileText,
-        _ => IconName::File,
-    }
-}
-
 /// A file in a message: its kind's icon, its name and size, and a way to download it.
 #[derive(IntoElement)]
 pub struct FileMessage {
@@ -412,15 +402,3 @@ impl RenderOnce for LinkPreviewCard {
             }))
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn a_file_name_picks_its_icon() {
-        assert_eq!(file_icon("lift.PNG"), IconName::Image);
-        assert_eq!(file_icon("notes.md"), IconName::FileText);
-        assert_eq!(file_icon("archive"), IconName::File);
-    }
-}