Thumbnail: Always ask before sending any binary/encoded attachments. Don't assume the recipient wants to see it, or that they can even detach and view the particular document format.
Context:
Sometimes we have a document, or even a large text-file, or the contents of a web page, that we wish to include as part of the message we are sending to someone. We could simply reference the document, but it might not be available to the recipient, or you don't know the proper citation. And you really would like them to see the contents of the document, and not just hear about it (it might even save time, or the sending of an extra message). However, you don't usually send email to this particular person, or maybe you've just never sent them attachments before, and you don't know how they might react to it.
Problem:
How should you convey the information in this document to the recipient?
Forces:
The recipient might be really interested in the information the document contains. It might even inspire them to take actions you think are beneficial (which may have been your intent all along). Or they might have no interest whatsoever and curse your name for wasting their valuable time.
Attachments, especially encoded attachments for documents created using word processors and other programs that create binary-format files, take up a substantial amount of space and typically cause the entire message to be much larger than your typical message that has no attachments.
Some of us have lots of disk space to burn and don't mind storing lots and lots of messages in our incoming mailboxes. OTOH some of us pay for email accounts hosted on someone else's machine. And its not uncommon for the hosting organization to have policies and/or limitations regarding the maximum allotted space for a user's incoming mailbox or "spool" file. If the mailbox file overflows, it can cause all subsequent messages to that user "bounce": the user may not ever see them, subsequent senders to the overflowed mailbox may receive a "bounce" message, which often reproduces the entire text of the bounced message.
If the message is attached in a MIME-compliant format, there is a good chance the recipient will be able to attach. But this isn't always true. Furthermore, even if the user can detach it, they may not have the software needed to correctly view the detached document. Or they may have the software, but the incorrect version of it that is required to view what was sent. This happens frequently with Acrobat-reader PDF files for example.
Even if the necessary software or software version is readily obtainable, if the receiver doesn't have it, they will need to spend a non-trivial amount of time downloading and/or installing the software.
Often times the recipient does have necessary software to detach and view the attached document, and lots of spare disk space, but they don't appreciate the amount of detailed information being sent: It inundates their mind far more than they ever wanted to know about the topic, causing information overload and often a great deal of resentment.
Recipient with slow Internet connection may not have adequate control of mail software; so they may not be able to download "headers only" or to download message bodies without attachments.
Recipient may be concerned about MalWare (IE: virus or worm) in attached executable files or macros.
Solution:
Unless you have prior knowledge about the recipients preferences and attitudes regarding email with encoded attachments and the topic in question, then don't ever include an attachment without first asking the recipient if they are interested in reading it. Try to give a brief executive summary of the document to pique their interest (and, if possible, provide a URL or citation so they can find it themselves), and then ask them if they would like you to email the document to them in a separate message.
Don't forget to let them know what format the document file is so they can determine whether or not they even have the requisite software to view it. Formats like MS-Word or postscript (or even PDF) may seem ubiquitous to those familiar with them, but not everybody has the corresponding word processor software (they may not even be using a compatible operating system - not everyone uses MS Windows).
Resulting Context: