It's not an idiom, it's a computer direction. "Opening" an attachment means clicking on the link and causing the attachment to upload to your computer.
That would depend on what the attachment was.
It means things are trustworthy.
To make a mistake
About to be sold, or given up.
say no to it
It means shut up!
flatter her to get something
To keep struggling and not give up.
he suddenly turned up (to appear, emarge)
to make something stronger ,,
It's not an idiom. To break camp means to break it up, to pack your things and leave the area. It can be used as slang, however, to mean a group "packing up" and leaving.
The idiom "crack someone up" means to make someone laugh or amuse them greatly.