fix(excel): Excel Export add mime type to work in Firefox, fixes #500

[ghiscoding]

• fixes #500

[codecov]

Codecov Report

Merging #501 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #501   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          148       148           
  Lines         9556      9557    +1     
  Branches      3192      3193    +1     
=========================================
+ Hits          9556      9557    +1     

Impacted Files	Coverage Δ
.../angular-slickgrid/services/excelExport.service.ts	100.00% <100.00%> (ø)

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