Parse error when trying to fetch true attachments

[KacperFKorban]

Parsing msg with attachment named Attachment[1.pdf and trying fetchTrueAttachments() throws an exception, because cid:['\"]Attachment[1.pdf['\"] is not a valid regex. (the square bracket isn't closed)

I created a PR with a fix for it #30.

[bbottema]

Apologies that it took so long for me to look at your problem and change. I've release your fix in 1.7.5, thank you for the effort, much appreciated!