Fix backslash and uppercase extension issue

When uploading a file, it added images like this:
`<img src="\/path/to/file">` 
So i substr the first letter (which is probably bad.)
when files have extensions like .PNG instead of .png, it fails to show the image in the editor, so i have it auto lowercase the filename before inputting the data.

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This is just a recommendation - feel free to implement this how you want :)