update content-disposition header to MIME-style

[eudore]

Content-Disposition in the Context.FileAttachment method is not a MIME style, but a slight performance response that can be avoided.

benchmark test file: https://github.com/eudore/eudore/blob/master/_example/textprotoHeader_test.go

[codecov]

Codecov Report

Merging #2512 into master will decrease coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2512      +/-   ##
==========================================
- Coverage   98.73%   98.45%   -0.28%     
==========================================
  Files          41       41              
  Lines        2367     1948     -419     
==========================================
- Hits         2337     1918     -419     
  Misses         17       17              
  Partials       13       13              

Impacted Files	Coverage Δ
context.go	97.50% <ø> (-0.63%)	⬇️
render/json.go	85.29% <0.00%> (-2.52%)	⬇️
response_writer.go	93.33% <0.00%> (-1.91%)	⬇️
debug.go	92.50% <0.00%> (-1.50%)	⬇️
utils.go	96.72% <0.00%> (-0.65%)	⬇️
recovery.go	97.14% <0.00%> (-0.36%)	⬇️
gin.go	99.00% <0.00%> (-0.15%)	⬇️
fs.go	100.00% <0.00%> (ø)
auth.go	100.00% <0.00%> (ø)
... and 32 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3100b7c...57829e5. Read the comment docs.

[thinkerou]

lgtm