Allow setting thumbnail mode when uploading

[hv0905]

This allow users to create an local thumbnail even if the image isn't a local image.

[codecov]

Codecov Report

Attention: Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.93%. Comparing base (799d6e1) to head (d87b2c0).

Files	Patch %	Lines
app/Controllers/admin.py	90.00%	1 Missing ⚠️
app/Controllers/search.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   76.41%   78.93%   +2.51%     
==========================================
  Files          29       29              
  Lines        1238     1253      +15     
==========================================
+ Hits          946      989      +43     
+ Misses        292      264      -28     

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[codeclimate]

Code Climate has analyzed commit d87b2c0 and detected 0 issues on this pull request.

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