Add support for jpg and png file extensions for small map

[ron-murhammer]

Address issue here: https://forums.triplea-game.org/topic/577/large-middle-earth-queries/112

Allow .jpg and .png file extensions in addition to .jpeg for small map images.

[codecov-io]

Codecov Report

Merging #3452 into master will decrease coverage by <.01%.
The diff coverage is 8.33%.

@@             Coverage Diff              @@
##             master    #3452      +/-   ##
============================================
- Coverage     21.99%   21.98%   -0.01%     
  Complexity     5915     5915              
============================================
  Files           832      832              
  Lines         71907    71912       +5     
  Branches      11584    11586       +2     
============================================
  Hits          15813    15813              
- Misses        54009    54014       +5     
  Partials       2085     2085

Impacted Files	Coverage Δ	Complexity Δ
...in/java/games/strategy/triplea/image/MapImage.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...rc/main/java/games/strategy/triplea/Constants.java	100% <100%> (ø)	3 <1> (+1)	⬆️
.../src/main/java/games/strategy/net/nio/Decoder.java	72.82% <0%> (-1.09%)	24% <0%> (-1%)

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