Attachment with UTF8 Characters as its Filename Appears Gibberish When Using "Download All" Feature

Steps to reproduce

1. Have Chinese-charactered filenames.
2. Attach these files into a JIRA instance.
3. Choose the option to download all attachments.
4. Open the downloaded ZIP file.
5. The result: all filenames with Chinese characters appears gibberish. See the screenshot below for more information.
   
6. However, the contents of the file remains intact.

Extra Notes

• also affecting attachment with umlaute characters in the filename
• file downloaded individually will have correct name
• Tested on both Chinese-locale and English-locale.
• Implemented all checks on UTF-8 encoding checks as per Knowledge Base article: http://confluence.atlassian.com/display/JIRAKB/Internalisation+and+Encoding+Troubleshooting