I recently implemented a custom processor that used the Exifr gem to
extract EXIF information from images uploaded to a Paperclip attachment.
Exifr's processor uses readbyte to parse the EXIF header, so it hit
errors when Paperclip's File-like object didn't have one. There's also a
test for this delegation.

I've also tidied up the README documentation for custom processors,
hopefully to be more clear than before. There was some duplicated
content between the "Post Processing" section and the "Custom Attachment
Processing" section, and those sections were separated in the file. I've
dedicated the "Post Processing" section to Paperclip's built in
thumbnailing processors, and made "Custom Attachment Processing" section
solely about writing & using your own custom processors. This should be
a more understandable progression, as built-in functionality is
discussed first & separately from extending Paperclip.k