How I can get the mime type of a file having its Uri?

You can try

ContentResolver cR = context.getContentResolver();
MimeTypeMap mime = MimeTypeMap.getSingleton();
String type = mime.getExtensionFromMimeType(cR.getType(uri));

Edit :

mime.getExtensionFromMimeType(cR.getType(uri)) 

returns -> “jpeg”

cR.getType(uri);

returns “image/jpeg” that is the expected value.