An attachment is a computer file or files place inside an email message. Email was originally designed to handle only plain text (no formatting, ie. bold, centering, etc.) which was transferred from one computer to another in a format called ASCII. ASCII is a standard across all computer types which makes e-mail universal. Today many modern email programs allow HTML (hypertext markup language) to be included in email messages which allow you to format your email messages with font sizes, bold, centre and such. The recipient must have an HTML capable Email reader to see the formatting otherwise they will get a document marked up with HTML tags.
To attach a document to an email message create a new email message and choose File Attachment from the menu. Most modern email programs place a button on the Toolbar usually in the shape of a paper clip that you can click. A dialog box will open that allows you to select the file that you want to attach. You can attach more than one file to a single email message. Most computers also allow you to drag and drop a file onto a new email message to attach it. Address the email message and send it. Don't move the original document until the message has been sent.
Most documents created on computer, such as word processing documents (Word, WordPerfect, etc.) or spreadsheet documents (Excel, Quatro, etc.) or graphic files (Corel Draw, Paint Shop, or .gif and .jpg files) are stored in their own unique binary code format. This binary format is determined by the program that the file is created with. Email documents are create as text files so in order to send a binary file or document via email, it must first be encoded into a text format and then attached to the email text message.
Such an 'encoded' document may end up looking something like this:
ugAAANAAAAALAAAAAgD//zQAQgBvAGQAeQAgAFQAZQB4AHQAAABCAG8AZAB5ACAA ANMFCwAAAQAACwDT0wQdAAATAAD//wUAsAQAmA0AEA4AaBAAK5sdANPUGxkAgAEAA AACKAooCGQDU1BpXAIABAQAIAAAAAAAAAAAAHAD0GlwSGgkAABEJAAAAYAAYEQAAE ZQBzACAATgBlAHcAIABSAG8AbQBhAG4AAAAAAAAAVABUAAAAVwDU1AILAAABAAELAN
These lines are all printable or lower-ASCII characters and can be sent via email. When it gets to your computer and you want to view the document, first it must be decoded or converted back to it's binary format before it can be opened. Today this is typically handled by your Email program.
There are several common encoding standards, the most popular being uuencoding, mime, and binhex. Not all email packages support even these three, let along all the non-standard ones. If someone sends you a document encoded in a format that your email program doesn't support the email program will not decode the attachment.
The file must be loaded into it's respective program to be viewed. That is, if the document was originally created in MS-Word you need MS-Word on your computer to work with the document. Most modern software suites can import documents from other suites, but typically it's best to have the correct matching program.
Most modern Email programs work hand-in-hand with your operating system to try and open the correct program required to view the document sent as an attachment. This is done by matching the file extension such as .jpg, .gif, .doc, etc... with a registered file type.
Viruses can also be transferred via email attachments. Because your email software handles the decoding of programs sent as attachments it is easy to infect your computer simply by opening an infected attachment. Always check email attachment with an anti-virus program before opening them.
There are several reasons why an attachment will not display properly or at all:
When sending an attachment you should think about whether the intended recipient has the right decoder and the right viewing program. When you are sending an attachment to someone you should always use a standard encoding process (the one that comes with your email program) and send the document in a format that the recipient can view. If in doubt, ask them first by email.
Macs users should be aware that most Windows user won't be able to decode a binhex file.
If the formatting isn't important it's better to copy your document from the original program then paste it into the body of a plain text email message rather than sending an attachment. Everybody's email software can read this ASCII format.