Underscore prefix on long attachment names

[hm122]

When I am using long names for mail attachments, the attachment name starts with an underscore, even though I did not set an underscore:

Content-Type: text/plain; name="_XXXXXXX_YYYYYYY_ZZZZZZZ_123456789.csv" Content-Transfer-Encoding: quoted-printable Content-Disposition: attachment;filename="_XXXXXXX_YYYYYYY_ZZZZZZZ_123456789.csv"

Do you know what might cause this problem?

[milo]

What contains the $file when you add it by addAttachment()? The name is sanitized here.

[hm122]

I have found out that this problem also depends on the mail client. The original email source looks like this:
Content-Disposition:` attachment; filename="
Filename_debug_customer_date_20160322_114204.csv"

While the file is correctly displayed in thunderbird, Outlook shows the file as "_Filename_debug_customer_date_20160322_114204.csv".

The file contains simple csv text data, the filename is set exactly as above, but without newline or spaces in front.

Following another example of with an empty file attached:

Short name (works perfectly fine):
Content-Type: application/x-empty
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="foobar.csv"

Long name (adds a new line and two white spaces or tab):
Content-Type: application/x-empty
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="
foobar_foobar_foobar_foobar_foobar_foobar_foobar_foobar_.csv"

[xsuchy09]

I can confirm this bug. For example PDF software is opening PDF file as attachment with %09 at the beginning of the filename - because of tab which is added in MimePart:append called from MimePart:encodeHeader which is called from MimePart:getEncodedHeader. When file is saving Ubuntu save it with - at the beginning. Attachment is added like:

Content-Disposition: attachment; filename="
	invoice-7lQYGxp6er2Dpb28ZdDvPOoML.pdf"

Solution is to add it like:

Content-Disposition: attachment; filename=
	"invoice-7lQYGxp6er2Dpb28ZdDvPOoML.pdf"

or don't fold header lines which is not specified in RFC. It is just good use case for terminal clients ofc.

More info in czech language:
https://forum.nette.org/cs/31371-email-prilozeny-soubor-a-podivne-09-pred-nazvem-souboru#p208896

[dg]

@xsuchy09 Thanks for the analysis and solution.