Some e-mail clients (also known as e-mail MUA) adhere to the new standard RFC2231 for attachment filename handling. Superb’s  webmail, Thunderbird 1.5, and some newer e-mail clients adhere to these new standards. The new standard requires that any spaces and special characters in the attachment filenames are converted to their MIME equivalent. For example, a file name my document.doc is converted to my_document.doc.

The problem is that some e-mail clients, including Outlook and some older clients still use the old standard RFC2047. As a result, they do not recognize the converted filename format and surprisingly renames those attachments to .BIN or .DAT

Solution: Use e-mail clients that support RFC2231 standard.

Workaround: If you cannot use newer e-mail clients or those that support RFC2231, you can avoid this problem by using filenames that do not contain spaces or special characters. For example, if you want to attach a file that has a space in the filename such as my document.doc, just rename it as my_document.doc.