Underscore prefix on long attachment names about mail HOT 4 CLOSED

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"

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@xsuchy09 Thanks for the analysis and solution.

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

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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

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