[frontend] Extension is not always set to .json (#7720)

[ValentinBouzinFiligran]

Proposed changes

• ".json" added at the end of the new file name
• mime type removed

Related issues

• Extension is not always set to ".json" in dashboard export, which lead to not be able to re-import without renaming the file #7720

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.31%. Comparing base (7d60d33) to head (628daf0).
Report is 28 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8746      +/-   ##
==========================================
+ Coverage   66.29%   66.31%   +0.01%     
==========================================
  Files         597      597              
  Lines       60958    60958              
  Branches     6267     6266       -1     
==========================================
+ Hits        40414    40422       +8     
+ Misses      20544    20536       -8     

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[ValentinBouzinFiligran]

mime type removed to be simpler as written in the lib doc