Known Issues
As discussed in issue 249 and elsewhere, you may encounter problems when not using the mount_on option in a mounted uploader. Due to bug/feature/unexpected behavior in ActiveRecord, when loading records from database using associations it might happen that ActiveRecord tries to assign String value to uploader column. If this happens you need to rename database column (for example use file_identifier instead of file) and specify new name as mount_on option when mounting uploader.
If you get an error such as "Attachment failed to be processed" ensure that you have installed the relevant image manipulation library before installing the associated gem.
Read this Stack Overflow page and try uninstalling any prexisting installations and deleting your cache.
brew remove imagemagick
brew --cache imagemagick
brew install -f imagemagick --disable-openmp