Allow streamy access to attachments

[lowjoel]

Currently CarrierWave only allows us to read an entire file into memory if we want to be backend agnostic. This is not possible for large attachments.

Tagged security because file uploads can be big, and can cause DoS.

[lowjoel]

@allenwq can add your findings here.

[cysjonathan]

large file is going to blow memory carrierwaveuploader/carrierwave#1780
Fix CarrierWave reads large files into memory carrierwaveuploader/carrierwave#2314
Should be fixed in CarrierWave >= 1.2.3

[zhuhanming]

In that case, this should be fixed, as the current version of carrierwave is 2.1.0.