update for supporting file binding

[p581581]

update for supporting multipart form and file binding

example:

type PhoptUploadForm struct {
	imgData    *multipart.FileHeader `form:"img_data" binding:"required"`
	ProjectID   string `form:"project_id" binding:"required"`
	Description string `form:"description binding:"required"`
}

ref: #1263

[codecov]

Codecov Report

Merging #1264 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1264      +/-   ##
==========================================
+ Coverage   98.63%   98.64%   +0.01%     
==========================================
  Files          41       41              
  Lines        2124     2145      +21     
==========================================
+ Hits         2095     2116      +21     
  Misses         18       18              
  Partials       11       11

Impacted Files	Coverage Δ
binding/binding.go	100% <100%> (ø)	⬆️
binding/form.go	100% <100%> (ø)	⬆️
binding/form_mapping.go	100% <100%> (ø)	⬆️

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[thinkerou]

@p581581 please fix conflict, thanks!

[thinkerou]

@p581581 please fix conflict, thanks!

[thinkerou]

@p581581 any updates? thanks!

[p581581]

I thought this pr is closed. Give me some time to fix the conflicts.

[thinkerou]

@p581581 OK, thanks! I will save the pr until you fix it.

[haoguo]

any progress of this feature? really need it. I could make some contribution to this if no one do it, @thinkerou

[p581581]

Thank you!! @thinkerou. I'm too busy to fix it, sorry about that.

[thinkerou]

@appleboy please help review the pull request, thanks!

[appleboy]

@thinkerou maybe add an example in gin-gonic/example repo or add tips in the documentation.

[thinkerou]

@appleboy OK, I add it.