It appears resource type "file" in 2.4 has now a different behavior than 2.3 when opening it in a new window or popup.

To reproduce in Moodle 2.4+ (Build: 20121214):

• create a course
• add a file resource, and upload a single pdf file
• configure the resource to be opened in a pop up
• save and return to the course home page
• now click on the resource name. Moodle displays a pop up containing the page with the resource description and the link "Click on <NameOfPdfFile.pdf> to view the file"

In Moodle 2.3 with the same settings the resource file pdf is opened and shown in the pop up directly. Tested also "New window" and the difference among 2.3 and 2.4 is the same.

On our testing environments, on the pop up windows the URLs appears as follows:
Moodle 2.4: domain.tld/mod/resource/view.php?id=2&redirect=1
Moodle 2.3: domain.tld/pluginfile.php/272/mod_resource/content/3/NameOfPdfFile.pdf

Not sure if this is intentional or not, anyway it could be worth a check.