Follow-up to r71944: Interoducing MimeMagic::improveTypeFromExtension() for two reasons:

Follow-up to r71944: Interoducing MimeMagic::improveTypeFromExtension() for two reasons:
a) avoid redundant inspection of file contents when validating uploads, caused by multiple calls to guessMimeType
b) deprecated obscure use of the file extension when guessing mime types, using an explicit call to improveTypeFromExtension() instead

Note that File::getPropsFromPath() will now return an additional field: $props['file-mime'] contains the mime type as determined solely from the file's content, $props['mime'] contains the type that was derived considering the file extension too.