core: handle image download cache

packages/hydrooj/package.json

@@ -28,9 +28,11 @@
         "koa-static-cache": "^5.1.4",
         "lodash": "^4.17.21",
         "lru-cache": "^6.0.0",
+        "mime-types": "^2.1.32",
         "minio": "7.0.17",
         "moment-timezone": "^0.5.33",
         "mongodb": "^3.6.11",
+        "nanoid": "^3.1.25",
         "nodemailer": "^6.6.3",
         "p-queue": "^7.1.0",
         "reflect-metadata": "^0.1.13",
@@ -50,6 +52,7 @@
         "@types/koa-static-cache": "^5.1.1",
         "@types/lodash": "^4.14.172",
         "@types/lru-cache": "^5.1.1",
+        "@types/mime-types": "^2.1.1",
         "@types/minio": "^7.0.9",
         "@types/mongodb": "^3.6.20",
         "@types/nodemailer": "^6.4.4",