feat(discord): Optimize segment.image sending

[dragon-fish]

#281 被我搞出了毛病，所以我重开了

[codecov]

Codecov Report

Merging #282 (adc3742) into develop (ad25f8c) will increase coverage by 0.16%.
The diff coverage is 98.30%.

@@             Coverage Diff             @@
##           develop     #282      +/-   ##
===========================================
+ Coverage    92.36%   92.52%   +0.16%     
===========================================
  Files           57       58       +1     
  Lines        10881    11255     +374     
  Branches      2233     2323      +90     
===========================================
+ Hits         10050    10414     +364     
- Misses         831      841      +10     

Impacted Files	Coverage Δ
packages/koishi-test-utils/src/app.ts	100.00% <ø> (+0.96%)	⬆️
packages/plugin-github/src/index.ts	95.65% <95.00%> (-0.12%)	⬇️
packages/koishi-core/src/adapter.ts	78.32% <100.00%> (+0.31%)	⬆️
packages/koishi-core/src/app.ts	97.72% <100.00%> (+4.59%)	⬆️
packages/koishi-core/src/context.ts	93.41% <100.00%> (-0.75%)	⬇️
packages/plugin-common/src/admin.ts	87.15% <100.00%> (+1.37%)	⬆️
packages/plugin-common/src/basic.ts	98.90% <100.00%> (+<0.01%)	⬆️
packages/plugin-common/src/handler.ts	100.00% <100.00%> (ø)
packages/plugin-eval/src/index.ts	93.10% <100.00%> (-0.34%)	⬇️
packages/plugin-eval/src/loaders/default.ts	70.21% <0.00%> (-29.79%)	⬇️
... and 14 more

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