Add support for a Wiki syntax to reference attachments

[katana@…]

As far as I know there is currently no way to reference an attachment using Wiki syntax. It would be quite handy since attachments are linked to the whole ticket and not to a specific comment.

I don't really have an idea at hand for a proper syntax but I'm sure someone would come up with something :)

[cboos@…]

See #944 for a proposed implementation (with patch).

[cboos]

I'll integrate this in the InterTrac related refactoring of the Wiki formatter.

The syntax I propose is:

• attachment:test.png -- link to the attachment page for that file
• image:test.png -- simple inlining of the referenced image
  Will be equivalent to the [[Image(attachment:test.png)]] macro call.
• attachment:attachment.1073.diff:#944 -- link to the attachment page for the attachment.1073.diff file attached to the ticket #944

[cmlenz]

I'd prefer not adding a link namespace for inlining images, but rather leave that to the [[Image()]] macro. Links should be links.

On a related note, I'd also like to deprecate the special handling of links with common image file extension names in 0.9.

[cboos]

I implemented the attachment: link syntax in [1868],
and dropped the support for image: links.
This will be left to the [[Image()]] macro.

Also, we need to define a proper way to know the source object
when processing wiki text... The current way is to rely on the
formatter.req.path_info information, which is not optimal.