Fix UF2 file identification so that it works

[whyitfor]

• [x] I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!) UF2 file identification was not working. This PR fixes this, and updates the tests to specifically test that identifcation works.

Please describe the changes in your request.

• Add a Magic Mime identifier for UF2 files
• Deletes custom Uf2FileIdentifier which was not working
• Updates UF2 tests with a test to assert that identification works
• Update UF2 unpack test to remove adding Uf2File tag (this was making the test pass even though the identifier wasn't working)

Anyone you think should look at this, specifically? @sjossi

[sjossi]

Good catch and thanks for the neat fix!