Allow modifying the MediaType of a layer during mutate.Append.

[dlorenc]

No description provided.

[codecov-commenter]

Codecov Report

Merging #761 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #761   +/-   ##
=======================================
  Coverage   79.02%   79.03%           
=======================================
  Files         102      102           
  Lines        4734     4736    +2     
=======================================
+ Hits         3741     3743    +2     
  Misses        550      550           
  Partials      443      443           

Impacted Files	Coverage Δ
pkg/v1/mutate/mutate.go	75.00% <ø> (ø)
pkg/v1/mutate/image.go	72.93% <100.00%> (+0.41%)	⬆️

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