Hi Abrar Ahmed,
Because you say it depends on the receiver, it's most likely a security measure of the mail server of the other organisation.
Common solutions are to remove dots and spaces in the file name and to not use excessive length names, but thats clearly not applicable.
I guess the MIME type is not set correctly. Could you try to set it to application/vnd.openxmlformats-officedocument.spreadsheetml.sheet instead of application/octet-stream? Not sure what integration you use, but I hope you are able to do this.
Another, less likely possibility could be that you have multiple attachments with the same name. I guess you can easily rule this out by checking the source of the email sent.
Good luck!
Email attachment