Allow arbitrary file extensions on Image panes

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #1489 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1489   +/-   ##
=======================================
  Coverage   86.07%   86.07%           
=======================================
  Files         141      141           
  Lines       15638    15640    +2     
=======================================
+ Hits        13460    13462    +2     
  Misses       2178     2178           

Impacted Files	Coverage Δ
panel/util.py	81.81% <ø> (ø)
panel/pane/image.py	80.41% <100.00%> (+0.27%)	⬆️

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