When a file is attached to any form, irrespective of the file name, it is showing the attachment name as null within the manage attachment popup as well as on the form(initially).
After reloading the form, attached filename shows up correctly.
The issue may occur if there is a "gr" variable being used (that is not wrapped in a function) within an ACL on the sys_attachment table or on the affected table (e.g. incident, change etc.).
This conflicts with the attachment code which uses a gr variable as well.
Change the variable name from 'gr' to any nonreserved(not used by the system at the backend) word.
Note: As a best practice, do not use "gr" are your glide record variable. Make a custom variable name that you know will not be in the system or wrap your script in a function and then that gr variable will not impact others.