Some custom fields with square brackets don't work

Title: Some custom fields with [] don't work

Short description: If you have custom fields that start with [] and next text, the custom field doesn't export the text you have in that custom field

Note: if you have ${[1234]} or ${Text[Brackets]} it's possible to export. Custom fields work. 

Steps to reproduce:

1. Create a custom field called "[KMD] NDC"
2. In one issue add some text to the custom field
3. Create a template that exports that custom field
4. Export the template in the issue you used in step 2

Actual Result: the custom field doesn't export the text you added.

Expected Result: the text should be exported.