refactor: express custom directives with standard markdown constructs

core/src/mime.rs

@@ -10,6 +10,9 @@ use std::path::Path;
 /// Returned when the extension is absent or unrecognised.
 pub const DEFAULT: &str = "application/octet-stream";
 
+/// Plain text, which is also what marks a file as inlinable ASCII art.
+pub const PLAIN_TEXT: &str = "text/plain; charset=utf-8";
+
 /// The media type to serve `path` as, by extension, compared case-insensitively.
 pub fn media_type(path: &Path) -> &'static str {
     let Some(extension) = path.extension().and_then(|e| e.to_str()) else { return DEFAULT };
@@ -34,7 +37,7 @@ const TABLE: &[(&str, &str)] = &[
     ("ico", "image/vnd.microsoft.icon"),
     ("avif", "image/avif"),
     // Documents and data
-    ("txt", "text/plain; charset=utf-8"),
+    ("txt", PLAIN_TEXT),
     ("gmi", "text/gemini; charset=utf-8"),
     ("css", "text/css; charset=utf-8"),
     ("html", "text/html; charset=utf-8"),
@@ -42,7 +45,7 @@ const TABLE: &[(&str, &str)] = &[
     ("json", "application/json"),
     ("toml", "application/toml"),
     ("pdf", "application/pdf"),
-    ("asc", "text/plain; charset=utf-8"),
+    ("asc", PLAIN_TEXT),
     // Audio and video
     ("mp3", "audio/mpeg"),
     ("ogg", "audio/ogg"),