You can post directly to the Main Stream in the same way you post to other communities: navigate to the Main Stream click into the box that says Write a post, give it a title or a citation, use Attach to add any additional files, then click Post.  

Accepted File Types

The following types of files can be attached to posts:

• Images:  .gif, .jpg, .jpeg, .png
  
• Video:  .avi, .m4v, .mov, .mp4, .mpg, .wmv
  
• Audio:  .mp3, .wav
  
• Documents:  .doc, .pdf, .rtf, .txt