Try to sniff the mime type based on the body content

Synchronous body reader now exposes a peek() function to get the first few bytes
from the response body. This will be no less than 100 bytes (assuming the body
is that big), but could be more. Streaming API, via res.next() continues to work
as-is even if peek() is called.

Introduce Mime.sniff() that detects a few common types - the ones that we care
about right now - from the body content.

Author: karlseguin

src/browser/browser.zig

@@ -435,24 +435,19 @@ pub const Page = struct {
 
         log.info("GET {any} {d}", .{ url, header.status });
 
-        const ct = blk: {
-            break :blk header.get("content-type") orelse {
-                // no content type in HTTP headers.
-                // TODO try to sniff mime type from the body.
-                log.info("no content-type HTTP header", .{});
-
-                // Assume it's HTML for now.
-                break :blk "text/html; charset=utf-8";
-            };
-        };
+        const content_type = header.get("content-type");
 
-        log.debug("header content-type: {s}", .{ct});
-        var mime = try Mime.parse(arena, ct);
+        const mime: Mime = blk: {
+            if (content_type) |ct| {
+                break :blk try Mime.parse(arena, ct);
+            }
+            break :blk Mime.sniff(try response.peek());
+        } orelse .unknown;
 
         if (mime.isHTML()) {
             try self.loadHTMLDoc(&response, mime.charset orelse "utf-8");
         } else {
-            log.info("non-HTML document: {s}", .{ct});
+            log.info("non-HTML document: {s}", .{content_type orelse "null"});
             var arr: std.ArrayListUnmanaged(u8) = .{};
             while (try response.next()) |data| {
                 try arr.appendSlice(arena, try arena.dupe(u8, data));