Am I crazy?

if foo in bar is not valid syntax.

defining an instance method in Application Controller should make it
available in the child controllers.

However, as this information is constant and probably related to your
data model rather than your presentation, I would suggest using a
constant in the appropriate model.

For instance, if you have an Image model, you would do:

class Image

...

FILE_TYPES = %w( jpeg tiff gif eps ).freeze

and then use it as:

Image::FILE_TYPES.include? f.type

Brian: Stay cool, you are okay.

Others: Jesus, why in the hell most guys out there are not listening
careful to a simple, reasonable and important question?? This is not
about IRB, or how to do it in a different way. And suggestions like
"read a good book" are not helpful at all, btw. This is about name
scopes, and what we need is a precise definition here.

David: What you say matches the documentation and it makes sense.
Furthermore, you WROTE a good book.

In the Rails application I am currently working on I have created a
method in the application controller. I cannot call it in any other
controller, neither as a private nor as a public method.

After having created a completely new Rails application I did the same
thing. Now I could call it successfully. Private or public, it does
not make any different.

With other words: Rails is behaving inconsistent. Bad news. And it
might very well explain why it worked the other day in Brians
application but stopped doing so 3 days later.