How to attach file to a github issue?

You upload it somewhere and add the link in a comment. GitHub's Issues is rather primitive and doesn't allow attaching files.

Update: You can post images to GitHub issues now. The easiest way is to copy the image (right click, Copy image) and then paste it into the text box where you describe the issue.

OR

Just drag and drop

As of December 7, 2012, you can attach images by drag/drop or use a file chooser. See https://github.com/blog/1347-issue-attachments for more details.