HTTP Sampler/Files upload tab - auto detect MIME type of selected file

**eR@SeR** ([Bug 65027](https://bz.apache.org/bugzilla//show_bug.cgi?id=65027&redirect=false)):
Hello,

When one or more files are selected to upload in HTTP Sampler/Files upload tab, the file path is generated but the MIME type isn't. IMO, it is better to auto-detect MIME type of selected file, than to type it manually every time.

Jmeter 5.4.1 9d2ba68
Microsoft Windows 10 Pro 64-bit
java version "1.8.0_251"

Created attachment [auto detect mime type.jpg](https://apache.github.io/jmeter-bugzilla-attachments/27/65027/37648/auto%20detect%20mime%20type.jpg): auto detect mime type

OS: All