Fix content-type in ThumbnailGenerator

[wojtha]

Fixes #78

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f1d480f) 100.00% compared to head (829d992) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #80   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         1828      1828           
=========================================
  Hits          1828      1828           

Impacted Files	Coverage Δ
sqlalchemy_file/processors.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[jowilf]

Thank you for your contribution