Improve image detection

Here's a re-roll of the patch in #2 against the current HEAD. I expect the tests will still fail for the reasons I described in #4, because nothing has been done to fix that.

The last submitted patch, 5: 3155325-5-type-guesser.patch, failed testing. View results →
- codesniffer_fixes.patch Interdiff of automated coding standards fixes only.

Corrected two @var declarations to use the current version of Symfony.

Have to also update guess() to be guessMimeType() for D10. Because guess() was deprecated in D9.1 this change will also work with our current minimum core version of 9.3.

Note the fact that the D9 tests did not catch this is a direct result of our lack of testing for certain key methods. In this case, guess() is used in MimeMailFormatHelper::mimeMailFile and MimeMailFormatHelper::mimeMailMultipartBody, but neither of those are tested. See:
#3145400: Add test cases for MimeMailFormatHelper::mimeMailFile() →
#3145401: Add test cases for MimeMailFormatHelper::mimeMailMultipartBody() →

OK, #8 fixed the new errors related to guess(), so we're back to needing to fix the one test failure described in #4.

Re-rolled against HEAD and included a fix for #4.

This issue is blocking a D10 release.

The last submitted patch, 10: 3155325-10-type-guesser.patch, failed testing. View results →
- codesniffer_fixes.patch Interdiff of automated coding standards fixes only.

Well that did fix the issue from #4 but it caused another issue, which I will now have to look into.