How to attach previously uploaded files?

Click on previously uploaded in the comment box: 

You will see a window where previously uploaded images are shown. When you hover over them, they will zoom out1 and there you will see two system folders, CLIP и MOBILE2 — where all the files ever copied or downloaded are automatically stored. Click on the desired file, and then click on Attach:

 

Also, attach previously uploaded files from other projects. In order ot do this, just simply switch to another project by clicking on the title of the current one3: