Add QOI to .gitattributes

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Needed by SixLabors/ImageSharp#2446

[codecov]

Codecov Report

Merging #39 (9bfbffe) into main (f0d7ed2) will not change coverage.
The diff coverage is n/a.

@@        Coverage Diff         @@
##           main   #39   +/-   ##
==================================
  Coverage    19%   19%           
==================================
  Files         4     4           
  Lines       346   346           
  Branches     85    85           
==================================
  Hits         68    68           
  Misses      278   278           

Flag	Coverage Δ
unittests	19% <ø> (ø)

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