Correct me if I am wrong, but if you have filenames of the form user_nnnnn.jpg, and you rename them to the form user_md5.jpg, the only "loss of data" that could occur is if a user has uploaded the same file under two different names?
Yes, that's exactly the point morgon is making when warning against the suggestion to hash on file content instead of file name in one of the ancestors of this post.

In reply to Re^10: Renaming an image file by JavaFan
in thread Renaming an image file by Anonymous Monk

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