egui_extras `image` doesn't use image plugins

**Describe the bug**
egui_extras uses the image::guess_format with ImageReader::with_format, this make it unable to utilize image codecs added by plugins.

**To Reproduce**
Try to load an image supported by a plugin for `image`.

**Expected behavior**
Support for plugins.

**Solution**
Use ImageReader::new instead, as it doesn't have this issue. And remove file extension and mime type checking

Locate the egui_extras image-loading code that calls image::guess_format with ImageReader::with_format, then read how file extensions and MIME types are checked there. Reproduce the issue with an image format supplied by an image plugin, and verify that plugin-supported images load successfully without those checks.

Written by the indexing model from the issue text.