--Boundary.1633693627
Content-Type: application/pdf; name="TRPB_1_1678965312.pdf"
Content-Disposition: attachment; filename="TRPB_1_1678965312.pdf"
Content-Transfer-Encoding: base64

Thunderbird uses the internet style media types when deciding what action to take with an attachment. In is styling the file extension is not only irrelevant, it is optional. Only to the windows operating system is a file extension more than a decoration. Registered text media type are listed here https://www.iana.org/assignments/media-types/media-types.xhtml#text Despite common terminology, attachments are not files. They are mime encoded text in the boday of an email. They only become files when you elect to do something with them, at which point the MIME text in the email is converted to a file and written to the file system. This is the beginning of a mime encoded block for a PDF file --Boundary.1633693627 Content-Type: application/pdf; name="TRPB_1_1678965312.pdf" Content-Disposition: attachment; filename="TRPB_1_1678965312.pdf" Content-Transfer-Encoding: base64 When Thunderbird processes this document it will create a file names as specified and it will take action on the file based on the settings for that Content Type: I suggest you check your incoming mail if correctly encoded by the sender using the content type: text/CSV. (Ctrl+U opens the message source and you now know what sort of formatting to look out for to determine the file type). My guess is the file is not correctly encoded as CSV, but as something associated with notepad, like plain text.