Changed the `attachment:` link resolver syntax to:
{{{
attachment:<file>:<the resource>
}}}
e.g. attachment:sample.png:ticket:123
This syntax is homogeneous with the new syntax for comment: (see r3825 and the [comment🎫2168:7 original proposal]).
This new syntax style:
{{{
<resolver>:<detail>[:<optional resource>]`
}}}
is more general and is better suited for eventually locating "nested" resources (like a comment for an attachment).
