Below you can find a list of 'strange behaviours' / 'known problem' you can encounter when using Alfresco Mobile for Android and external 3rd party Application.
Gallery Application Blank/Black Image
The official 'Gallery application' uses an internal rendition (kind of internal thumbnail) to display an image file. The process to create this rendition is asynchronous and can be done when the file is protected ie the thumbnail rendition can be created during the period of time where the file is unencrypt. That explain why sometimes the file might appears as blank/black image but the file is correctly unprotected and readable by others applications. To avoid this behaviour you can use any other image viewer (which don't have this cache rendition)
3rd Party Application
Sometimes it may be impossible to open a protected file with a 3rd party application. An error message like 'file doesn't exist or cant be opened' may appears. In this case, it's possible to decrypt the file before opening it. Just click to the right dark triangle next to a file and select the action 'Decrypt' Next you will be able to tap on the file and open it with a 3rd party reader.