thank you for your feedback!
We are currently improving the file previewer. These improvements include a real full screen mode and a zoom feature for images.
Opening files in other apps can be done by pressing the share button in full screen mode or you mark the file in the browser and press the share button.
Also editing files should be possible. For text files you can use our text editor and press save after editing or you open the files in an other app by clicking on them. After you returned to Boxcryptor your saved changes should be uploaded.