pkg/v1/mutate: fill in mediaType for OCI

[maisem]

Fixes #1235

[codecov-commenter]

Codecov Report

Merging #1236 (7abcfb6) into main (d9bfbcb) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1236      +/-   ##
==========================================
- Coverage   73.80%   73.77%   -0.03%     
==========================================
  Files         111      111              
  Lines        8191     8183       -8     
==========================================
- Hits         6045     6037       -8     
  Misses       1549     1549              
  Partials      597      597              

Impacted Files	Coverage Δ
pkg/v1/mutate/image.go	69.23% <100.00%> (-0.50%)	⬇️
pkg/v1/mutate/index.go	78.99% <100.00%> (-0.85%)	⬇️

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