The allowed file types come from the web.config setting for AllowedMediaFileExtensions

but even that allows files you probably should not use. The main purpose of the media attachments is to support the iTunes extensions in the feed.

You don't need to use media attachments if you just want to link to files, you can do that right in the content using the link toolbar item in the editor allows browsing the server and linking to files.

As far as size limitations, by default IIS limits it to about 4MB but you can allow larger files, see the article Supporting Large File Uploads

Hope that helps,

Joe