Replace underscores in attachment titles for AAIB import

This is a work-around for a bug in Kramdown which causes the following

[a_1.pdf](http://example.com/a_1.pdf)

to produce:
1: <p><a rel="external" href="http://example.com/a_1.pdf">a_1.pdf</a></p>

but got:
1: <p>[a<em>1.pdf](http://example.com/a</em>1.pdf)</p>