Use Content-Disposition: attachment for .eml files

Steps to reproduce

1. Attach a .eml file to a JIRA issue
2. Using IE11, click the attachment link

Expected results

IE11 downloads the .eml file as an attachment.

Actual results

IE11 attempts to display the file inside the browser instead of downloading it.

Notes

The HTTP headers when clicking the link include Content-Disposition:inline. Chrome ignores this and downloads the file correctly, but IE11 respects the header. Ideally all .eml files should download as files. HTML files are actually sent with Content-Disposition:attachment even though a browser could display the content inline.