By default, the 
upload icon appears instead of the 
image icon if you add more file extension to the default ones in authorized extensions site settings.
The default file extensions: jpg, jpeg, png, gif, heic, heif, webp
This will show the image icon for users.
If you add more file extensions to it: e.g.: jpg, jpeg, png, gif, heic, heif, webp, pdf
I added pdf and this will show the upload icon for users.
There are two site settings for this:
-
authorized extensions
(this setting change the uploads for all users) -
authorized extensions for staff
(this setting only change the uploads for staff users)
These are the default settings 
But as Canapin’s said you may check your plugins / theme components too. Maybe there is a conflict which override the default upload behaviour.