[Bug]: 7zip headers warning with nextcloud 31 generated zip files

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

i am trying to download multiple files from a public shared folder, which in turn creates a zip file and that file gets downloaded. when trying to extract it with 7zip (or nanazip from ms store) i get header errors warning. this does not happen with nextcloud 30 and prior.

Steps to reproduce

1. upload multiple files into a folder.
2. share the folder.
3. open the shared folder link and select all files.
4. click download.
5. try to extract the downloaded file with 7zip or nanazip.

Expected behavior

no warnings

Nextcloud Server version

31

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.4

Web server

Nginx

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

Upgraded to a MAJOR version (ex. 31 to 32)

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

{
    "system": {
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
        ],
        "dbtype": "mysql",
        "version": "31.0.0.18",
        "overwrite.cli.url": "http:\/\/files.ivanjkara.net",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "memcache.local": "\\OC\\Memcache\\APCu",
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "filelocking.enabled": true,
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 6379
        },
        "upgrade.disable-web": true,
        "maintenance_window_start": 4,
        "log.condition": {
            "apps": [
                "admin_audit"
            ]
        },
        "log_type": "file",
        "logfile": "\/data\/nextcloud.log",
        "log_type_audit": "file",
        "logfile_audit": "\/data\/nextcloud.log",
        "simpleSignUpLink.shown": false,
        "maintenance": false,
        "loglevel": 2,
        "trashbin_retention_obligation": "30, 35",
        "files.chunked_upload.max_size": 20971520,
        "app_install_overwrite": [
            "checksum"
        ]
    }
}

List of activated Apps

Enabled:
  - activity: 4.0.0
  - admin_audit: 1.21.0
  - app_api: 5.0.2
  - bookmarks: 15.1.0
  - bruteforcesettings: 4.0.0
  - checksum: 1.2.5
  - circles: 31.0.0-dev.0
  - cloud_federation_api: 1.14.0
  - comments: 1.21.0
  - contactsinteraction: 1.12.0
  - dav: 1.33.0
  - deck: 1.15.0
  - federatedfilesharing: 1.21.0
  - federation: 1.21.0
  - files: 2.3.1
  - files_downloadlimit: 4.0.0
  - files_external: 1.23.0
  - files_pdfviewer: 4.0.0
  - files_reminders: 1.4.0
  - files_sharing: 1.23.1
  - files_trashbin: 1.21.0
  - files_versions: 1.24.0
  - firstrunwizard: 4.0.0
  - groupfolders: 19.0.3
  - impersonate: 2.0.0
  - logreader: 4.0.0
  - lookup_server_connector: 1.19.0
  - nextcloud_announcements: 3.0.0
  - notes: 4.11.0
  - notifications: 4.0.0
  - oauth2: 1.19.1
  - password_policy: 3.0.0
  - privacy: 3.0.0
  - profile: 1.0.0
  - provisioning_api: 1.21.0
  - recommendations: 4.0.0
  - related_resources: 2.0.0
  - richdocuments: 8.6.2
  - serverinfo: 3.0.0
  - settings: 1.14.0
  - sharebymail: 1.21.0
  - sociallogin: 5.9.2
  - spreed: 21.0.0
  - support: 3.0.0
  - survey_client: 3.0.0
  - systemtags: 1.21.1
  - text: 5.0.0
  - theming: 2.6.1
  - twofactor_backupcodes: 1.20.0
  - twofactor_totp: 13.0.0-dev.0
  - updatenotification: 1.21.0
  - user_status: 1.11.0
  - viewer: 4.0.0
  - webhook_listeners: 1.2.0
  - workflowengine: 2.13.0
Disabled:
  - dashboard: 7.11.0 (installed 7.9.0)
  - encryption: 2.19.0
  - photos: 4.0.0-dev.1 (installed 2.5.0)
  - suspicious_login: 9.0.1
  - twofactor_nextcloud_notification: 5.0.0
  - user_ldap: 1.22.0
  - weather_status: 1.11.0 (installed 1.9.0)

Nextcloud Signing status

no errors

Nextcloud Logs

irrelevant

Additional info

No source file or test is named. Start by reproducing the public-shared-folder multi-file download on Nextcloud 31 and compare the generated ZIP with Nextcloud 30, then trace the ZIP generation and download entry point. Done means 7zip and Nanazip extract the downloaded archive without header warnings, with a regression test covering the behavior.