Opera, when configured with the "Determine action by MIME type" option disabled, interprets an object as an HTML document even when its MIME Content-Type is…

Opera, when configured with the "Determine action by MIME type" option disabled, interprets an object as an HTML document even when its MIME Content-Type is text/plain, which could allow remote attackers to execute arbitrary script in documents that the user does not expect, possibly through web applications that use a text/plain type to prevent cross-site scripting attacks.