When you try to attach file from internal storage, GMail writes an error to the log:
ERROR/Gmail(...): file:// attachment paths must point to file:///mnt/sdcard.
E-mail application would show you the attached file even if it didn't physically exist.
As for an external storage, documentation says that:
Every Android-compatible device supports a shared "external storage" that you can use to save files. This can be a removable storage media (such as an SD card) or an internal (non-removable) storage.
That means you don't have to worry about device not having an external storage at all. Still, external storage can be unavailable at times. Refer to
Storage Options | Android Developers