feat attachment filename support utf8

[a-wing]

attachment filename support utf8

Content-Disposition: attachment; filename="new_filename.go"
Content-Disposition: attachment; filename*=UTF-8''new%F0%9F%A7%A1_filename.go

golang.org/x/exp/utf8string need golang 1.18

https://datatracker.ietf.org/doc/html/rfc5987
https://stackoverflow.com/questions/93551/how-to-encode-the-filename-parameter-of-content-disposition-header-in-http
https://stackoverflow.com/questions/53069040/checking-a-string-contains-only-ascii-characters

[codecov]

Codecov Report

Merging #3071 (405fee4) into master (3757142) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3071   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          41       41           
  Lines        3090     3100   +10     
=======================================
+ Hits         3052     3062   +10     
  Misses         26       26           
  Partials       12       12           

Flag	Coverage Δ
go-1.13	98.77% <100.00%> (+<0.01%)	⬆️
go-1.14	98.61% <100.00%> (+<0.01%)	⬆️
go-1.15	98.61% <100.00%> (+<0.01%)	⬆️
go-1.16	98.61% <100.00%> (+<0.01%)	⬆️
go-1.17	98.51% <100.00%> (+<0.01%)	⬆️
macos-latest	98.77% <100.00%> (+<0.01%)	⬆️
nomsgpack	98.75% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	98.77% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
context.go	97.90% <100.00%> (+0.01%)	⬆️
utils.go	97.00% <100.00%> (+0.19%)	⬆️

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