How to Fix the Problem of Attachments Being Renamed in Thunderbird When the Names Exceed 64 bytes Size?

You might have noticed that the names for attachments sent using Thunderbird sometimes gets renamed, mainly the extensions. For example, some of the documents type format (like Word doc or docx) changes to .DAT or .BIN. Some versions of Thunderbird display the names correctly, but the mails with attachments sent using Thunderbird when viewed by other programs like Outlook may not appear correctly. This is most commonly seen with attachment filenames with double-byte characters or other text encoded using Unicode, as opposed to ASCII.

The Issue – Attachment Renamed in Thunderbird

Usually when the filenames exceed 64 characters limit, there is also a problem. But this particular issue is caused by the filenames exceeding 64 bytes limit, even if the characters are under 64 limit. Because of double-byte characters, this limit can exceed and this causes problems with displaying names because they are being replaced by other multiple parameters.

Not all email clients support the same MIME standard protocol or version for handling the names of the files attached with emails. The latest version of Thunderbird supports RFC2231 parameter, but if you view that email in Outlook, it won’t be displayed correctly but Outlook supports RFC2047.

Some versions of Thunderbird may display the names correctly, but the other versions may not. Therefore, if this is a consistent problem for you, you must change the default parameter to RFC2047 to handle the filenames in MIME email headers. It’s also a necessity for users who often send and receive emails in Unicode standard (non-English).

This cannot be done by the main Thunderbird interface based settings page. For this, you might need to open a more advanced category of modifications that are not commonly accessed and might also be risky if you are not sure of what you are doing. Therefore, proceed with caution while following the tutorial below.

First, open the config-editor.

1. Click on the “Settings” icon on the top right side. It looks like this – ‘≡’
2. Then open “Options”
3. In “Options,” scroll down to the bottom and click “Config Editor” and click “I accept the Risk.”

Now, you are inside the ‘Config Editor.’ Make any changes here carefully, as it might modify Thunderbird’s behavior irreversibly.

1. Type “mail.strictly_mime.parm_folding” inside the search box and hit enter.
   
2. Double click on the entry and then enter the numeric value as 0 or 1. 0 or 1 refers to RFC2047 protocol for MIME based emails. If it is 2, it means it will use RFC2231. The value 3 also uses RFC2231 but name param is encoded by RFC2047. So, for the issue of the attachment filenames being renamed to .DAT or .BIN, you must change it either 0 or 1 to force Thunderbird to use only RFC2047 parameter.
   
3. Now, restart Thunderbird and hopefully your problem with attachment file names will be resolved.

Also Read: How to fix corrupt MSF file

Conclusion

Remember that if the Thunderbird sent emails (with attachments) when viewed with other clients are still being renamed, the reason might be with how those other clients view files and their names and how they handle the MIME parameters. Those scenarios can only be fixed through the settings of those clients in which you are viewing the emails with attachments.