FileInfo.com - The File Extensions Resource

.B64 File Extension

File Type	Base64 MIME-Encoded File
File Format	File
Common?	No
File Description	Binary file encoded using base64 encoding; often used to convert text data into base64 data; enables more reliable transmission of e-mail attachments and other information; similar to a .MIME attachment.
Program(s) that open b64 files	Mac: Smith Micro StuffIt Deluxe for Mac Windows: Smith Micro StuffIt Deluxe for Windows; Corel WinZip

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/b64