Are the files in fact identical, only the file extension may be incorrect?
If so, then a (Regular Search) 'Same content' search would find those duplicates.
(It would not know the "correct" file extension, only that the files themselves were in fact duplicates.)
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If the files are the same
shot, but are of physically different formats, then... I'm not sure, offhand, particularly

.
What (Image Mode ?) settings have you enabled for your search (in this case)?