Fix attachment issues

Fixes https://github.com/PrivateBin/PrivateBin/issues/315
The attachment viewer is also used for storing to be uploaded
attachments, which caused some confusion in handling them.
I also tried to add some comments clarifying it as it seems to work.

Additionally I fixed the issue that you could submit an empty paste and
it was not rejected.

.gitignore

@@ -1,7 +1,9 @@
 # Ignore server files for safety
 .htaccess
 .htpasswd
-cfg/conf.php
+cfg/*
+!cfg/conf.sample.php
+!cfg/.htaccess
 
 # Ignore data/
 data/