Add multipart example

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (dev@f0aff07). Click here to learn what that means. The diff coverage is n/a.

@@          Coverage Diff           @@
##             dev     #136   +/-   ##
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  Coverage       ?   92.54%           
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  Files          ?       27           
  Lines          ?     1315           
  Branches       ?        0           
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  Hits           ?     1217           
  Misses         ?       98           
  Partials       ?        0

Impacted Files	Coverage Δ
R/BackendRserve.R	71.31% <ø> (ø)

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[lintr-bot]

inst/examples/multi-part/app.R:11:14: style: Use =, not <-, for assignment.

​echo_handler <- function(request, response) {
             ^

inst/examples/multi-part/app.R:20:7: style: Use =, not <-, for assignment.

​  cnt <- request$get_file("csv") # 'csv' from the upload form field
      ^

inst/examples/multi-part/app.R:22:6: style: Use =, not <-, for assignment.

​  dt <- read_csv(cnt)
     ^

inst/examples/multi-part/app.R:28:7: style: Use =, not <-, for assignment.

​  tmp <- tempfile()
      ^