Whether you used a web-based e-mail service like Hotmail, Yahoo Mail, or GMail, or a program installed on your home computer like Microsoft Outlook, you might sometimes need to send a file to a friend by e-mail.

I'll show you how to do this in Hotmail; the steps for Yahoo Mail, GMail, and Microsoft Outlook are similar. A file that is sent with an e-mail is called an attachment; in all e-mail services or programs, when you're composing a new e-mail message, you should be able to see an option to add an "attachment" to the message.

Below is the screen that appears when you're composing a new message in Hotmail; to add an attachment, click the "Attach" link:

When you click that link, you will be prompted to locate the file you want to send. (The image below is what appears if you do this on a Windows computer; on a Macintosh or other systems it will look different).

Most Windows programs store files under the "My Documents" folder, which you can locate by clicking "My Documents" in the left-hand side of that screen. Microsoft Word, for example, saves new Word documents in the "My Documents" folder; most modern graphics programs will save pictures by default in the "My Pictures" folder, which is inside the "My Documents" folder. Once you've located the file that you want to attach, click "Open". (Which is slightly confusing, because you're not actually opening the file, you're selecting it to be attached to your message, but that's just how it works.)

Now, after a brief interval during which the file is being uploaded to the Hotmail site, you will see the message displayed with the name of the file that has been attached:

Note the comment above the filename: "Total size: 19.5 KB of 10.0 MB". A message sent from Hotmail can only contain a total of 10 megabytes' worth of attachments. That's 10 megabytes, total, for all the files attached to a single message, not 10 megabytes per file. Other services have different limits; GMail, for example, lets you send up to 20 megabytes. Most common types of files, such as pictures and Microsoft Word documents, are far below that size limit, so you usually won't need to worry. (The only common type of file that might sometimes be above the size limit, would be a video file or an MP3 audio file; check the file size before trying to send those types of files by e-mail.) But while on that subject, a couple of things to keep in mind about sending files by e-mail, especially large ones:

In addition to the limit on outgoing file sizes imposed by your e-mail service (10 megabytes using Hotmail; 20 megabytes using Gmail), there will usually also be a limit on the size of incoming file attachments on the recipient's end. If the file that you try to send the recipient, is larger than that limit, the e-mail message will be returned to you as undeliverable. As a rule of thumb, just assume that the limit on attachment sizes that they can receive, is about the same as the limit on attachments that you can send.

Sending large files -- larger than, say, 1 megabyte -- may be annoying to people who weren't expecting them. (This is especially true if the user uses a home e-mail program like Microsoft Outlook to download their mail, because they'll sometimes have to download the entire message, along with the enormous attachment, just to get to the rest of their mail. Users of web-based services like Hotmail, on the other hand, will be less annoyed since they can just delete the message without downloading the attachment.) If the file is a picture or a video, consider uploading it to your Facebook profile or putting it on YouTube and giving people the link to find it there, rather than sending them the whole file.

And that's it! The message should send like a normal e-mail message, and you can sit back and wait for your friend's replies about how impressed they were with your latest efforts at nature photography. (Isn't that the tree in your backyard? Did you take the photo through the window, or did you at least go outside?)

Bennett Haselton is a technology and political blogger and can tell you all about how to get around your Internet filters.