2. If you require to assign Custom permission to the users, You can assign the same form SharePoint Site. To assign Custom permissions for a Page: Navigate to the page >> Click on Site Settings >> Advanced Permissions >> Select the user and Modify the Permissions. In case of a new User Navigate to the page >> Click on Site Settings >> Advanced Permissions >> Click on "New" option to add User to that particular Page and assign the required permission.
Here in this case the user was trying to upload 60 MB PowerPoint file onto the SharePoint site. Since we are in a hosted Model, We have certain Limitations in place. Our Hosted SharePoint site has a Time-Out limit of 50MB/20Min. This means, user will be able to upload attachment upto 50MB within a timespan of 20 Mins.