Some inline images render incorrectly on some email clients

Inline images aren't being recognised properly on some email clients. Specifically, if the resource comes from a plugin (and thus the Content Name of the image contains a ":" character) rendering will fail. This is probably because of some picky clients following http://tools.ietf.org/html/rfc6838#section-4.2 very closely, which says that ":" is actually not a valid character in the content name.

Reproduction steps:

1. Install Thunderbird and add your email account for Confluence
2. Assign a task to yourself on a page (ensure the "notify on my own changes" setting is enabled under User Settings)
3. Look at the email in Thunderbird - it should appear as the attached screenshot, with 2 attachments and the images not rendered inline.