Changed the markdown-extension pattern from !<name> to <name>: because
this will be much more readable in Markdown editor which don't
support these extensions.

They will then just render a text followed by a colon followed by a link:
<name>: <File-or-Folder-link>

Example:
It's no longer !pdf(Headline)(files/some-file.pdf)
It is now pdf: (Headline)(files/some-file.pdf)