When update of a checkbox in issue, all attachments are deleted and lost.

[wxiaoguang]

• Gitea version (or commit ref): 1.15.0+rc3
• Git version: builtin
• Operating system: Debian 10 / Docker
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes: https://try.gitea.io/wxiaoguang/testtest/issues/1
  • No

Description

Steps:

1. create an issue with a checkbox and an attachment
2. click the checkbox
3. the attachment is deleted (BUG)