Hello,
one of our clients uses matomo (I do not know which version exactly), and some HTTP responses fail when downloading reports, because of charset issues in the Content-Disposition header.
Here's a hex dump of one of those responses:

00000000      43 6f 6e 74 65 6e 74 2d 44 69 73 70 6f 73 69 74         Content-Disposit
00000010      69 6f 6e 3a 20 61 74 74 61 63 68 6d 65 6e 74 3b         ion: attachment;
00000020      20 66 69 6c 65 6e 61 6d 65 3d 22 45 78 70 6f 72          filename="Expor
00000030      74 20 5f 20 4d 61 69 6e 20 6d 65 74 72 69 63 73         t _ Main metrics
00000040      20 5f 20 44 65 63 65 6d 62 65 72 20 31 33 2c 20          _ December 13,
00000050      32 30 32 30 20 e2 80 93 20 4a 61 6e 75 61 72 79         2020 – January
00000060      20 31 31 2c 20 32 30 32 31 2e 63 73 76 22 0d 0a          11, 2021.csv"..
00000070      54 72 61 6e 73 66 65 72 2d 45 6e 63 6f 64 69 6e         Transfer-Encodin
00000080      67 3a 20 63 68 75 6e 6b 65 64 0d 0a 43 6f 6e 74         g: chunked..Cont

right after "2020", there's the – character, which is an en dash encoded as e2 80 93 in UTF8.

According to https://tools.ietf.org/html/rfc6266#section-4, when using the filename="" format, the name between double quotes should be (https://tools.ietf.org/html/rfc2616#section-2.2) in ISO-8859-1 charset, or in RFC 2047 format, like this: =?iso-8859-1?q?this is some text?= (for what it's worth, I never see anything in that format lately)

If the filename must include UTF-8 characters, it should use the filename*="" option, like this: UTF-8''%c2%a3%20and%20%e2%82%ac%20rates (cf https://tools.ietf.org/html/rfc5987#section-3.2.2 )
(the exact format is defined in https://tools.ietf.org/html/rfc5987#section-3.2.2 )

Unfortunately, I do not control this deployment of matomo, so my ability to test patches is limited, but I can request further information.

maybe related to #9580