Hmm... I would use Astro File Manager which is free in the Marketplace.
Once installed, go to your gmail app, compose a test email, menu key, attach, then you should have the option to either open Astro or Gallery. Select Astro, then find your file and attach and send off. Gmail has a file size limiter of 3 MB I believe, however using Astro you can send a file as big as 11-12 MB. Anything bigger than that will crash the Gmail app.
GriffMH, this should solve your problem for attaching non-media related files.
Flawlez427, give it a shot and if it still doesn't work, it could be a sync issue or maybe even crazy enough, your phone is waiting until you get a better connection to send the attachments? If you could provide a bit more info as to the type of files you're sending and size, it might help provide a better solution.
HTH
DD