Fix importing servers in Firefox for Android

[acelaya]

Closes #1130

This PR changes the import servers file input from accept="text/csv" to "aceppt=".csv".

The reason is that, with the former, Firefox for Android would not allow picking valid CSV files if they have spaces in their name.

The solution is not perfect, as now Firefox for Android allows to pick any file, but that's because it does not really support the accept attribute in file inputs. See https://caniuse.com/?search=input%20type%20file%20accept

With this change, Chrome for Android also allows to pick any file, but that was already its behavior before.

On desktop browsers the filtering works as expected, with the only limitation that you no longer can pick CSV files that don't have the CSV extension, and you can pick non-CSV files if they have the csv extension. But those are probably edge cases.

In any cases, Shlink handles the parsing of the file and displays a proper error message if an invalid file is picked, so this should put us in a better situation.

[github-actions]

Preview environment

https://shlinkio.github.io/shlink-web-client/feature/fix-firefox-import/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.59%. Comparing base (b399523) to head (0e0ea3f).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1135   +/-   ##
========================================
  Coverage    85.59%   85.59%           
========================================
  Files           55       55           
  Lines         2277     2277           
  Branches       306      306           
========================================
  Hits          1949     1949           
  Misses          23       23           
  Partials       305      305           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.