I´ve got it. There were two different Problems
The first Issue was answered by CodeDiving. I had to use the Authority from the provider-declaration for the getUriForFile call. Using an other class caused the NullPointerException.
I´ve tried to get a file from filesDirectory, but in my file_path I declared only a path to cache Directory. I changed it to 'files-path' and it worked. This error caused the IllegalArgumentException.