fix: Respect custom filer and thumbnail storage settings

[fsbraun]

Description

This PR fixes issue #1412: It respects custom filer private and public storage settings.

Related resources

• thumbnail is missing... [BUG] #1412

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Merging #1413 (b850c62) into master (d473d20) will increase coverage by 0.17%.
Report is 1 commits behind head on master.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1413      +/-   ##
==========================================
+ Coverage   74.83%   75.00%   +0.17%     
==========================================
  Files          75       75              
  Lines        3449     3449              
  Branches      554      554              
==========================================
+ Hits         2581     2587       +6     
+ Misses        698      694       -4     
+ Partials      170      168       -2     

Files Changed	Coverage Δ
filer/fields/multistorage_file.py	88.34% <ø> (ø)
filer/templatetags/filer_admin_tags.py	89.38% <75.00%> (ø)
filer/__init__.py	100.00% <100.00%> (ø)
filer/urls.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[vinitkumar]

@fsbraun Is it possible to write tests to verify the regression and the fix?

[marksweb]

LGTM 👍

[fsbraun]

@vinitkumar You're right: I'll add a test! The missing test is the reason we missed this in the first place.