mime: parse media types that contain braces

[jub0bs]

This CL fixes a bug introduced by CL 666655: isTokenChar would no longer
(but should) report true for '{' and '}'.

Fixes #76236

[gopherbot]

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[gopherbot]

Message from Sean Liao:

Patch Set 1: Code-Review+2 Commit-Queue+1

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Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-11-10T21:45:30Z","revision":"af5ccfcb9c8d737a05af7ca667646abe27997a74"}

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Message from Sean Liao:

Patch Set 1: -Commit-Queue

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Message from Go LUCI:

Patch Set 1:

This CL has passed the run

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Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result+1

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Message from Damien Neil:

Patch Set 1: Code-Review+2

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