All-things-Google blog Google Operating Systems offers an excellent primer on working with Gmail attachments, which the author says can be confusing. Here's a particularly great tip:
It's a good idea to select the files you want to attach before writing your email, because Gmail starts to upload them immediately, saving you precious time.
The author also refers you to helpful attachment-related scripts (like this one) and extensions and warns you against using Gmail for file storage (which he says could result in a locked account). Great stuff here for Gmail novices and experts alike.
Gmail Attachments [Google Operating System]