You need to create a custom implementation for the forum posts. You have to implement a button that will open a dialog with RadUpload control in it. Then if there is a file uploaded you need to save in a custom database table the post ID and some data for the uploaded file. Then each attachment could be a link to the file system directory where the file has been uploaded. Once an user clicks on the link he/she will be able to download/open the file.
In Sitefinity when you are replying to an item you use /Sitefinity/ControlTemplates/Forums/EditPost.ascx
template. Here you need to implement the RadUpload control and process the data.
The default module
control, so another option is using the built-in dialogs for inserting Images and Documents where you can upload and link to items.
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