Format messages in validates_format_of

Error messages from validates_format_of can't currently include the rejected value the way validates_inclusion_of and validates_exclusion_of can. This patch enables that.

The case where we want this is in validating a file attachment based on a name pattern. If the attachment doesn't have a known extension, we render back a file field for each failed upload, but we can't pre-fill file inputs, so we want the message to tell the user which filenames were rejected.