Issue Summary
A user attaching an image which has transparency in it as a thumnail in JIRA gets a broken image.

This appears correctly however if the image is clicked on for preview.

Steps to reproduce

• Create an issue.
• Add an attachment made up of an image file that has some transparency.
• Attach it in a comment as a thumbnail

  !example.PNG|thumbnail!

Expected Result
Image shows up like every other image added as a thumbnail.

Actual Result
Image is broken and would only display it's content correctly when a user clicks on it to preview.