You may drag and drop file attachements inline with the body of your post in the WYSIWYG editor.
Once uploaded your attachment can be seen in the preview pane if you do not have it hidden.
You may also add approved file types as attachments to emails and send them directly to a given category as a new topic. See the Email and Notification FAQ for emailing new topics from your email client.
Currently supported file types for attachment purposes are as follows:
The max attachment size is 10 MB as of 9/20/2016