Extend context.File to allow for the content-dispositon attachments via a new method context.Attachment

[emman27]

Extends the context.File method to allow the Content-Disposition header to be automatically set within this scope. This allows downloads of the file as attachments, allowing the server to specify the filename.

References:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition

Resolves #1243

[emman27]

Failure has nothing to do with the PR. Some bad gateway in the vendoring. Could someone trigger a rebuild / look into the bad gateway?

[thinkerou]

similar #1304 , it's ok, LGTM

[codecov]

Codecov Report

Merging #1260 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1260      +/-   ##
=========================================
+ Coverage    98.5%   98.5%   +<.01%     
=========================================
  Files          41      41              
  Lines        2070    2073       +3     
=========================================
+ Hits         2039    2042       +3     
  Misses         19      19              
  Partials       12      12

Impacted Files	Coverage Δ
context.go	98.37% <100%> (+0.01%)	⬆️

---

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 2dd3193...0eb1ee5. Read the comment docs.

[thinkerou]

@appleboy please help to review the pull request, thanks!

[appleboy]

Before the implementation. I just use the following code.

	c.Header("Content-Description", "Simulation File Download")
	c.Header("Content-Transfer-Encoding", "binary")
	c.Header("Content-Disposition", "attachment; filename="+fileName)
	c.Header("Content-Type", "application/octet-stream")

	c.File(filePath)