Using square bracket in file name will cause the generated link to be broken after upload

Issue Summary

Using square brackets in the file name will cause the generated link to be broken after upload.

Environment

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Steps to Reproduce

1. Create an attachment with square bracket (" [ " & " ] ").
2. While commenting on an issue, upload the attachment.
3. You will see the link is broken as shown in Actual Results.

Expected Results

Actual Results

Notes

• The file gets uploaded nonetheless.
• This is no filename spaces bug regression as an attachment with spaces in their name attached fine:
  

Workaround

Workaround 1
Use another markup syntax but escaping the square bracket with a backslash symbol:

[filename to be displayed by escaping the square bracket | link to the file by copy-paste from the attachment section] 

Example:

[some file with \[square bracket\].msg|http://baseURL/secure/attachment/12345/some%20file%20with%20%5Bsquare%20bracket%5D.msg]

Workaround 2
Upgrade to JIRA 7.13.4 (tested alright) and above.