restricting upload content type half working

paperclip converts the files correctly (they change extension and would
probably be served correctly), but it does not change the file's
filename or (which is the problem here) the file's content type.
suggestions are welcome
Restructuring and added validation according to content_type

+ validations are required since paperclip version 4.1.x
+ moved ImageAttachments to separate file in order to load ImageAttachment
constant.
ImageAttachment attachments are always converted to png files

implemented corresponding updater before_save hook-in.
Minor only update new images to png

introduced minor sanity check for before_save hook-in.
changed callback to after_post_process
changed back to before_save hook in and testing for file existence
strengthened condition for overriting content type and file name.

Checking for whether created_at date for this file already exists,
which is a workaround for already existing sketches.
Removed unused processor file

implemented similar approach by introducing a before save hook in,
which saves us from introducing an own paperclip processor.
Removed file_contents require

Postprecessor is no longer needed
Fixed rspec tests
Merge pull request #1 from fadendaten/jpg_only

Convert images to png files