This could happen if the text format of the message is set to “rich text”.
To change the format of a specific message, go to “Options”, and in the format frame, select “Html” (or any other format except “rich text”).
To change the default format for all messages, go to “Tools”->”Options”->”Mail format”, and select the desired format in the message format list box.
Hope this helps.
It also could be your antivirus. Are these attachments which you have previously sent successfully? If it is not your antivirus, some attachments are blocked by default. Here is a list.
You can also unblock them in the registry, just take this step with great caution.