refactor(server): use more specific HTTP status codes

[tessus]

Description

This change returns a more appropriate status code in 2 cases instead of a generic 500 code:

• UnsupportedMediaType (if the mime type is on the blacklist)
• Conflict (if the file already exists)

Motivation and Context

closes #261

How Has This Been Tested?

• cargo test
• fixtures

Changelog Entry

### Changed

- Return appropriate HTTP status code instead of 500

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added (in this case changed) tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 82.35%. Comparing base (4987cfe) to head (075fc84).

Files	Patch %	Lines
src/paste.rs	60.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   82.32%   82.35%   +0.02%     
==========================================
  Files          11       11              
  Lines        1228     1224       -4     
==========================================
- Hits         1011     1008       -3     
+ Misses        217      216       -1     

Flag	Coverage Δ
unit-tests	82.35% <60.00%> (+0.02%)	⬆️

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[orhun]

Thanks!