Add a Type field to Attachments

[wfarner]

Closes #312

Signed-off-by: Bill Farner bill@docker.com

[codecov-io]

Current coverage is 68.89% (diff: 73.68%)

Merging #316 into master will decrease coverage by 0.01%

@@             master       #316   diff @@
==========================================
  Files            27         27          
  Lines          1441       1453    +12   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            993       1001     +8   
- Misses          356        359     +3   
- Partials         92         93     +1   

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[chungers]

You beat me to it!! Thanks for this PR!

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "attachment-type" git@github.com:wfarner/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354030992
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[chungers]

Lgtm