diff --git a/packages/hydrooj/src/handler/home.ts b/packages/hydrooj/src/handler/home.ts
index 4e4c31e02..2ffbf471b 100644
--- a/packages/hydrooj/src/handler/home.ts
+++ b/packages/hydrooj/src/handler/home.ts
@@ -422,7 +422,7 @@ class HomeAvatarHandler extends Handler {
         } else if (this.request.files.file) {
             const file = this.request.files.file;
             if (file.size > 8 * 1024 * 1024) throw new ValidationError('file');
-            const ext = path.extname(file.originalFilename);
+            const ext = path.extname(file.originalFilename).toLowerCase();
             if (!['.jpg', '.jpeg', '.png'].includes(ext)) throw new ValidationError('file');
             await storage.put(`user/${this.user._id}/.avatar${ext}`, file.filepath, this.user._id);
             // TODO: cached avatar
diff --git a/packages/hydrooj/src/lib/mime.ts b/packages/hydrooj/src/lib/mime.ts
new file mode 100644
index 000000000..f5a12e9ff
--- /dev/null
+++ b/packages/hydrooj/src/lib/mime.ts
@@ -0,0 +1,8 @@
+import { lookup } from 'mime-types';
+
+export default function mime(file: string) {
+    const identifer = file.toLowerCase();
+    return (['.in', '.out', '.ans'].some((i) => identifer.endsWith(i)))
+        ? 'text/plain'
+        : lookup(file) || 'application/octet-stream';
+}
diff --git a/packages/hydrooj/src/model/storage.ts b/packages/hydrooj/src/model/storage.ts
index f6ab19873..c9cd51f12 100644
--- a/packages/hydrooj/src/model/storage.ts
+++ b/packages/hydrooj/src/model/storage.ts
@@ -1,9 +1,9 @@
 import { extname } from 'path';
 import { escapeRegExp } from 'lodash';
-import { lookup } from 'mime-types';
 import moment from 'moment-timezone';
 import { nanoid } from 'nanoid';
 import type { Readable } from 'stream';
+import mime from '../lib/mime';
 import * as bus from '../service/bus';
 import db from '../service/db';
 import storage from '../service/storage';
@@ -20,9 +20,7 @@ export class StorageModel {
     static async put(path: string, file: string | Buffer | Readable, owner?: number) {
         const meta = {};
         await StorageModel.del([path]);
-        meta['Content-Type'] = (path.endsWith('.ans') || path.endsWith('.out'))
-            ? 'text/plain'
-            : lookup(path) || 'application/octet-stream';
+        meta['Content-Type'] = mime(path);
         let _id = StorageModel.generateId(extname(path));
         // Make sure id is not used
         // eslint-disable-next-line no-await-in-loop
@@ -103,9 +101,7 @@ export class StorageModel {
         );
         const meta = {};
         await StorageModel.del([dst]);
-        meta['Content-Type'] = (dst.endsWith('.ans') || dst.endsWith('.out'))
-            ? 'text/plain'
-            : lookup(dst) || 'application/octet-stream';
+        meta['Content-Type'] = mime(dst);
         let _id = StorageModel.generateId(extname(dst));
         // Make sure id is not used
         // eslint-disable-next-line no-await-in-loop
diff --git a/packages/hydrooj/src/plugin-api.ts b/packages/hydrooj/src/plugin-api.ts
index 06a8e0296..ed15c5bb8 100644
--- a/packages/hydrooj/src/plugin-api.ts
+++ b/packages/hydrooj/src/plugin-api.ts
@@ -37,6 +37,7 @@ export { postJudge } from './handler/judge';
 export { Collections } from './service/db';
 export { Service, Context } from './context';
 export { buildContent } from './lib/content';
+export { default as mime } from './lib/mime';
 export { default as rating } from './lib/rating';
 export { default as avatar } from './lib/avatar';
 export { default as rank } from './lib/rank';
diff --git a/packages/ui-default/components/preview/preview.page.ts b/packages/ui-default/components/preview/preview.page.ts
index 912b13e0f..da7e195c7 100644
--- a/packages/ui-default/components/preview/preview.page.ts
+++ b/packages/ui-default/components/preview/preview.page.ts
@@ -121,7 +121,7 @@ export async function previewFile(ev?, type = '') {
     const link = $(ev.currentTarget).find('a').attr('href');
     if (!link) return null;
     type ||= ev.currentTarget.getAttribute('data-preview');
-    const ext = filename.split('.').pop();
+    const ext = filename.split('.').pop().toLowerCase();
     if (['zip', 'rar', '7z'].includes(ext) || filesize > 8 * 1024 * 1024) {
       const action = await new ActionDialog({
         $body: tpl.typoMsg(i18n('Cannot preview this file. Download now?')),