So you're using web based email and trying to send an email with an
attachment using their web interface.
That type of email applications lives on the mail server - not on your
system. That means that there's no mail setting on your end to adjust.
Visit the Help section for each email account. Read their policies on
attachments: size limits, types of files, etc. Make sure the files you're
trying to send is an allowed type and that their size does not exceed the
limits. If you continue to have trouble, contact their tech support.
The person you are mailing to may have limits imposed by their mail host in
regards to attachments. If that is the case, uploading the file and giving
them a link to download is a reasonable approach to exchange a file.