Android Question From where appears this ".back" - File?

Hi,

in my App I'm adding several files (png, txt and one Sqlite-Database) to a single zip file (with Archiver).

The zip contains files like this:
1205849_8-20141211_200247_sign.png

Suddenly there is this file:
1205849_8-20141211_200247_sign.png.back

In this case the first one is empty - the second one contains the data of the png.

Does anybody know why this .back-File is created and why the data of the origin is empty?
Is it created by the Archiver?