When using WordPress 3.5 (tried with 2 different installations) and latest Attachments version 3.0.4. I can't seem to attach PDF, DOC or TXT files.
When attaching, I can SEE all types of files in the new Media pop-up window, but after selecting for example one pdf file and clicking "Attach", nothing happens. The Attachments field still shows only "Attach files here!" message and "Attach files" button.
But, when selecting the image file as attachment, then everything works as it should and the attachment is visible with the 3 fields for Title, Caption and Copyright showing and attachments working/showing in the loop. Oh, and yes, the "filetype" property in the functions.php is set tu null.
Strange thing is, on one of the 2 installations I upgraded from the 1.6.2. version and the migrated PDF attachments from older version are visible (in admin and on the webpage).