Photos

From the discussion thread:

Click on the paperclip to the left of the message editing area
Click on “Photo library” to choose a photo from your gallery or “Camera” to take a photo directly
Choose the format you want to send the photo in
Add a caption if necessary
Click on "Publish" at the top right corner

Files

From the discussion thread:

Other
From another application :

Click on the "Share" button
Citadel Team will automatically be displayed in the list of applications to which you can share your information
Choose the room in which you want to post, either by clicking on one of the rooms displayed in the drop-down list, or by typing its name in the search box
Validate by clicking on "Ok"

Share an image or a file