In Rails, `ActionDispatch::Http::UploadedFile` has an accessor to access
a `Tempfile` object. However, `Rack::Test::UploadedFile` does not
provide that, but provinding the `#path` method instead. To be able to
support both, we have to check for existance for `#tempfile` method,
then fallback to `#path` method.

Fixes #807

Fixes #809

It's not working correctly as expected, so let's revert this for now.

This reverts commit 51bb0f9f.

Fixes #768

Fixes #786

Needs work for S3 Attachments.

Fix AttachmentContentTypeValidator to work with multiple values

Current validation check in AttachmentContentTypeValidator is simply wrong, it will add an error if any of the allowed_types fails comparison with value, so it will pass only if value is either empty or equals to each and every one of allowed_types.

Add test to check that validation passes if even one of content types match.

Rails 3.0.x doesn't support new migration style

This reverts commit b7bfdec3.

Note: find_each doesn't support order because it already specifies it.
And it happens to be 'id' as well.

minor typo in explanatory rdoc-comment