Hi Christian,
thanks a lot for addressing this!
Maybe it's possible to convert the default filename to the nearest ascii equivalent instead of just replacing it with an underscore.
For example: 'föö.html' => 'foo.html' instead of 'f__.html',
'fòôbàř.txt' => 'foobar.txt' instead of 'f_____.txt'.
I am not confident in what's the best way to do this - I usehttps://github.com/danielstjules/Stringy#toascii for such purposes in my own applications.

Best regards, Yannik

Why couldn't we useRFC 2231?
See also this old "paper" on the topic:http://greenbytes.de/tech/tc2231/
Doing transliterations works for western languages but is quite limited in the general case, I'm not really fan of this approach...

@nicolas-grekas Encoding a string according to RFC 2231 leads to% characters in the resulting string, doesn't it? This would conflict with how themakeDisposition() method of theResponseHeaderBag is working (seehttps://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/HttpFoundation/ResponseHeaderBag.php#L256) (but maybe we need to fix it there). What do you think?

@xabbuh Your PR is awesome! Hopefully this will be merged into Symfony.

@nicolas-grekas@xabbuh Can we find something that works for everyone here? I'd like to merge this one ASAP if possible.

If we removed the limitation of not being able to have the percent character in the fallback filename, we could use RFC 2231 afaics as@nicolas-grekas proposed. But I am not sure why this check was added initially.

@xabbuh Do you have time to finish this one? If not, just let me know and I will finish it for you.

@fabpot@nicolas-grekas I pushed an update.

👍

Thank you@xabbuh.

@fabpot Will this be merged into the more current releases of symfony?

@Yannik Yes, we regularly merge lower branches into all higher maintained branches.