Private Messages: Enable attachments

For service-oriented communities it's a regular occurrence to move a conversation to private messages in order to obtain customer information or data. Right now it is not possible to attach images (the WYSIWYG editor isn't activated for private messaging), PDFs or any other file types to a private message.

We would like to be able to have attachments for the standard file types (jpg, png, pdf, txt) enabled to private messages.