When viewing the summary tab of an iPhone device in iTunes, clicking on the candy color capacity chart will cycle the Audio, Video, and Photos measurements through space used on the device, number of items, and (for music and video) total playing time.
Since the iPhone, unlike an iPod, does not do disk mode, one cannot store files in memory and attach them to an email on the iPhone (as I used to do on the Treo). I needed to be able to store certain files that I would be emailing to many recipients in the course of a job.
The solution is to use IMAP as the mail protocol (either a vanilla IMAP server, .Mac or, it looks like Yahoo Mail on the iPhone uses IMAP). Once you're syncing via IMAP, creat a new folder on the IMAP server (via Mail app on the Mac); I called mine AttachmentFiles. Next, email yourself the files you want to use, and then move the received email to the new folder. Sync up with your iPhone, and you will see the new folder and the email therein.
To send those files, just choose the email that contains the one you want, and forward it with the option to include attachments.