you need to access the webtop configuration panel:
https://your.nethserver.tld/webtop
And log in with “admin” credentials (not admin@yourdomain.tld)
In the panel that appears to you, on the left column, click on the webtop logo icon
on the window that appears, in the middle column, select “Properties (system)” then, on the right panel, click on the “Add” button and select “com.sonicle.webtop.mail”
in the box with the cursor, type “attachment.maxfilesize” and in the box immediately to the right set the maximum size of attachments in bytes (Example 10MB = 10485760)