It seems that the withSpace method of AbstractContentFinder does not set the contentContainerId. This results in a null value being passed to the AttachmentContentTypeApiSupport.getAttachments method which results in a NullPointerException when it tries to call the asLong() method on the null value.
I am using Confluence 7.0.1 so perhaps this has already been fixed?
[relevant stacktrace]
Cause
java.lang.NullPointerException
at com.atlassian.confluence.api.impl.service.content.typebinding.AttachmentContentTypeApiSupport.getAttachments(AttachmentContentTypeApiSupport.java:98)
java.lang.NullPointerException
at com.atlassian.confluence.api.impl.service.content.typebinding.AttachmentContentTypeApiSupport.getAttachments(AttachmentContentTypeApiSupport.java:98)
at com.atlassian.confluence.api.impl.service.content.finder.ContentFinderFactory$ContentFinderImpl.internalFetchAttachments(ContentFinderFactory.java:149)
at com.atlassian.confluence.api.impl.service.content.finder.ContentFinderFactory$ContentFinderImpl.fetchMany(ContentFinderFactory.java:134)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)