Part of the design philosophy of iOS is to hide the file system from the user. Company webmail attachment system assumes that you have access to the file system. I don't think that there is any way to make them play nice, though I can imagine some convoluted workarounds.

iOS documents that you might want to attach to an e-mail "reside" in their respective applications. Open the document in that application and tap the action button (box with arrow exiting it) and select e-mail (assuming that it is an option). The problem is that Joe Line Guy doesn't (yet?) have the ability to access company e-mail through a stand alone e-mail app, only the web. So you could e-mail the attachment to your FedEx.com address from another e-mail address that you have set up on your iOS device, then forward that to the intended recipient from within the company webmail system.

Apple, it just works... unless they think you shouldn't have it, in which case, it does not work.

I do not have a company iPad. If somebody that does knows whether or not e-mail apps can access company e-mail, let us know.