Make checking attachment path clashes optional

Hi,

As the attachment clash checker is still generating false positives (#1143) despite being modified to look at attachment paths rather than URLs, I thought it would be useful to add an option to disable it - code and test attached.

Alternatively, since it was originally introduced to address unsafe default settings that have been fixed since v3.0, perhaps it's safe to completely remove? If you'd prefer, I can submit a PR to do that instead...

Cheers,
Simon