Currently there is no way to enter a macro argument including e.g. comma (@,@), which is always treated as an argument separator - see source:trunk/lib/redmine/wiki_formatting/macros.rb#L41. This effectively prevents using e.g. the @thumbnail@ macro for some images when renaming attachments is not an option. We'll be happy to submit a patch once there is some agreement on how to approach this.

Suggestion:

• Allow double quotes for argument value delimiting.
• No quotes = old behavior.
• Special characters within quotes can be encoded as HTML entities (similarly to @name="value"@ HTML element attributes).

Makes sense?

Rationale:

• @!@ turns off Redmine-interpreted text formatting constructs - but probably not suitable within the constructs.
• Text formatting docs say e.g. double quotes can be used when version name contains spaces
• Users tend to use double quotes anyway, e.g. @
  マクロ thumbnail の実行中にエラーが発生しました (添付ファイル foo.jpg が見つかりません)
  @ and they hardly realize that the quotes actually become part of the argument value.
• When I have a version called @My "Version"@, I can refer to it as @version:"My "Version""@ within Textile. Does not work with Markdown though.