🎉 It works

go.mod

@@ -0,0 +1,3 @@
+module codeberg.org/danjones000/slackmojidl
+
+go 1.22.2

main.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path"
+
+	"codeberg.org/danjones000/slackmojidl/models"
+)
+
+func die(v any) {
+	fmt.Fprintln(os.Stderr, v)
+	os.Exit(1)
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		die("Missing JSON file")
+	}
+
+	if len(os.Args) < 3 {
+		die("Missing directory")
+	}
+
+	dir := os.Args[2]
+
+	by, err := os.ReadFile(os.Args[1])
+	if err != nil {
+		die(err)
+	}
+
+	list := models.List{}
+	err = json.Unmarshal(by, &list)
+	if err != nil {
+		die(err)
+	}
+
+	for _, emoji := range list.Emoji {
+		dl(dir, emoji)
+		saveJson(dir, emoji)
+	}
+}
+
+func dl(dir string, emoji models.Emoji) {
+	ext := path.Ext(emoji.Url)
+	outPath := path.Join(dir, emoji.Name+"."+ext)
+	out, err := os.Create(outPath)
+	if err != nil {
+		die(err)
+	}
+	defer out.Close()
+
+	resp, err := http.Get(emoji.Url)
+	if err != nil {
+		die(err)
+	}
+	defer resp.Body.Close()
+
+	_, err = io.Copy(out, resp.Body)
+	if err != nil {
+		die(err)
+	}
+
+	fmt.Println("Saved", emoji.Name, "to", outPath)
+
+}
+
+func saveJson(dir string, emoji models.Emoji) {
+	outPath := path.Join(dir, emoji.Name+".json")
+	out, err := os.Create(outPath)
+	if err != nil {
+		die(err)
+	}
+	defer out.Close()
+
+	enc := json.NewEncoder(out)
+	enc.SetIndent("", "    ")
+	enc.Encode(emoji)
+
+	fmt.Println("Saved data to ", outPath)
+}

models/emoji.go

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+package models
+
+type Emoji struct {
+	Name            string   `json:"name"`
+	IsAlias         uint16   `json:"is_alias"`
+	AliasFor        string   `json:"alias_for"`
+	Url             string   `json:"url"`
+	IsBad           bool     `json:"is_bad"`
+	Synonyms        []string `json:"synonyms"`
+	UserDisplayName string   `json:"user_display_name"`
+}

models/list.go

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+package models
+
+type List struct {
+	Ok            bool
+	Emoji         []Emoji
+	DisabledEmoji []any `json:"disabled_emoji"`
+	Paging        Paging
+}

models/paging.go

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+package models
+
+type Paging struct {
+	Count uint32
+	Total uint32
+	Page  uint32
+	Pages uint32
+}