Don't use deprecated Unicode::mimeHeaderEncode()

Deprecated code should cover in D10 or D9 compatibility issues.
Patch #2 applied successfully and working as expected.

Here is the updated patch. Please take a look.

Please read the issue description

> At the same time I believe that this could be classified as a bug because there is no need to encode valid MIME type returned by the MIME guesser service in the first place.

This is not specifically about deprecation. There is no need to do encoding here - look at what core does https://git.drupalcode.org/project/drupal/-/blob/10.1.0/core/modules/fil... I do not see a reason why this module would need to do anything different.

I believe the correct approach is from the original patch to remove the encoding. RTBC based on patch in #2