Idea Summary
For the three “display as” types “Native Dropzone”, “Inline Dropzone”, “Block Dropzone”… if you specify the attribute “Storage Type” “APEX_APPLICATION_TEMP_FILES” and make sure “Allow multiple files” is set to No….

The users are still able to upload multiple files (!) if they drag-and-drop the files from their operating system. 

Interestingly you can't drag-and-drop multiple for the “display as” of “Native File Browse”… but its not as pretty…

Bug reproduced in APEX 20.2 (maybe its been fixed since then?)

Use Case
This makes it impossible to use these “display as” types if you need the enforce that the user can only upload one file. Its clearly not intended and should be fixed. Thanks.