add image mimetypes

[tanan]

Describe the PR
add mime type for comsumes and aproduces

• image/png
• image/jpeg
• image/gif

Relation issue
#135

[codecov-io]

Codecov Report

Merging #136 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   88.12%   88.29%   +0.16%     
==========================================
  Files           5        5              
  Lines         842      854      +12     
==========================================
+ Hits          742      754      +12     
  Misses         60       60              
  Partials       40       40

Impacted Files	Coverage Δ
operation.go	91.83% <100%> (+0.29%)	⬆️

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[pei0804]

LGTM

[easonlin404]

@tanan please aslo help update document. 😉 Thanks your contribution.
https:/swaggo/swaggo.io/blob/master/declarative_comments_format/api_operation.md#mime-types