Image Info reads all info from the data fork of files, so files without resource forks (i.e. files from PC platforms) are fully readable. (However Image Info does check for the presence of resources if any given info is not found in the data fork.)
Image Info determines file types (whether "TIFF", "JPEG" etc) by reading the header info. This means that files types accidently edited or corrupted or files from non-Mac platforms can be read safely.
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