Try deleting handlers.json, the replacement for the obsolete mimeTypes.rdf.

Open the source (Ctrl+U) and look at the Content-type header for the attachment.

The content-type is application/octet-stream; which should work. When I send myself, though TB, a tab delimited file, TB assigns the same content-type and TB does nothing when I try to open the file attachment.

Also, I deleted handlers.jason and it did not resolve the issue.

I did a test: exported an address book in tab delimited .tab format and attached it to a message. The source indicates the Content-Type: text/plain. When I Open the attachment, I get the attached picture, which makes sense with my default text editor. A content-type: application/octet-stream is probably part of the problem:

http://kb.mozillazine.org/Actions_for_attachment_file_types#application.2Foctet-stream

Note that I have the preference mail.strictly_mime set to true in Config. editor, which forces 7-bit (quoted-printable) format, which might make a difference. Also, try sending yourself a message with a .tab attachment from an account that encodes them with the correct content-type, as not all providers handle attachments the same way.

I must have been the content-type. Since its the output of a script that I wrote, I converted the output to csv and it opens properly.

Thanks for your help.