[bugfix] determine mime-type to use during ffprobe evaluation stage, don't bother checking against file extension

closes #3496

[x] I/we have read the GoToSocial contribution guidelines.
[x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
[x] I/we have not leveraged AI to create the proposed changes.
[x] I/we have performed a self-review of added code.
[x] I/we have written code that is legible and maintainable by others.
[x] I/we have commented the added code, particularly in hard-to-understand areas.
[ ] I/we have made any necessary changes to documentation.
[ ] I/we have added tests that cover new code.
[x] I/we have run tests and they pass locally with the changes.
[x] I/we have run go fmt ./... and golangci-lint run.

superseriousbusiness / gotosocial