Email: Add option to allow the download/upload of "potentially" insecure attachment

Imagine that you are in a bank, and forgot to take with you a very important
document. But you have that document in a zip file in your email account. So,
you switch the data connection, access the email, but you can't download your
zip file because the Email "thinks" is potentially unsafe. Yeap, come back
tomorrow!

This patch adds an option to enable/disable the upload/download of potentially
hazardous attachments (like zip, exe, apk, etc.). By default, "unsafe" email
attachments are unallowed, but a user which knows what he is doing can use the
setting to download those "unsafe" attachments.

No more big-brother to him who knows, ta-da..

Patchset2: Cleaned trailing whitespaces
Patchset3: Cleaned more trailing whitespaces

Change-Id: Ic93e9bf4c581b866dafa90cb0fb45d6ffa349660