Order the attachment extensions whitelist

In a blog post [0], Neil Williams suggested that people read the code
to find out the canonical list of attachment types that can be
uploaded.

It is difficult to scan the list quickly, because it is not ordered
alphabetically. This commit orders the whitelist.

[0] http://bit.ly/18b3ZgH
Merge pull request #665 from alphagov/extension-whitelist-ordering

Order the attachment extensions whitelist