A link to a file attachment may be created in a few different ways:
If using the WYSIWYG editor (TinyMCEPlugin), use the attach (paperclip) toolbar button to select an attachment on the "Insert link to attachment" tab and then press the "Insert link" button
Use the %ATTACHURL% macro (see VarATTACHURL). This is the full URL for attachments in the current topic. For example:
%ATTACHURL%/image.gif
To link to a file attachment in another topic, use the %PUBURL% macro (see VarPUBURL). For example: