There may be a utility to do this but I have using ResEdit to change the
file type and creator type of the images that STM won't recognize. Get a
copy of ResEdit and use the 'Get File/Folder Info' command under the
'File' menu. Then change the information in the 'Type' box to 'DSK5' and
the 'Creator' box to A2EM. Save your selection and STM should give the
file a new icon and recognize it.

Set:

Creator: A2EM
Type: DSK5

Then convert