[FIX] c.Request.FormFile maybe file, need close

[linfangrong]

#2104

func (r *Request) FormFile(key string) (multipart.File, *multipart.FileHeader, error)

File is an interface to access the file part of a multipart message. Its contents may be either stored in memory or on disk. If stored on disk, the File's underlying concrete type will be an *os.File.

[codecov]

Codecov Report

Merging #2114 into master will decrease coverage by 0.08%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2114      +/-   ##
==========================================
- Coverage   98.83%   98.74%   -0.09%     
==========================================
  Files          40       40              
  Lines        2233     2236       +3     
==========================================
+ Hits         2207     2208       +1     
- Misses         13       14       +1     
- Partials       13       14       +1

Impacted Files	Coverage Δ
context.go	98.66% <50%> (-0.44%)	⬇️

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