fix: thumbnails are too large in the chat box #818 (#819)

### What problem does this PR solve?

fix: thumbnails are too large in the chat box #818

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/pages/chat/chat-container/index.less

@@ -36,6 +36,9 @@
   // .referenceIcon {
   //   padding: 0 6px;
   // }
+  .thumbnailImg {
+    max-width: 20px;
+  }
 }
 
 .messageItemLeft {

web/src/pages/chat/chat-container/index.tsx

@@ -105,7 +105,10 @@ const MessageItem = ({
                     <List.Item>
                       <Flex gap={'small'} align="center">
                         {fileThumbnail ? (
-                          <img src={fileThumbnail}></img>
+                          <img
+                            src={fileThumbnail}
+                            className={styles.thumbnailImg}
+                          ></img>
                         ) : (
                           <SvgIcon
                             name={`file-icon/${fileExtension}`}