Improve big attachment handling

[trulex]

Is your feature request related to a problem? Please describe.
Sending attachments greater than 3 MB without providing optional size when calling messages.send() will fail.

Describe the solution you'd like
SDK should load file and figure out the size without relying on the client provided info.

Describe alternatives you've considered
Change api docs so it's clear that size is required when attachment is > 3MB.

Additional context
I spent some time debugging Error sending message: Error: max size of attachments for json is 3MB, received '6060096' to figure out size is actually required for SDK to figure out if multipart should be used. I haven't provided size since it's an optional argument and it's not clear in the docs it must be provided for this to work. After providing size it works as expected.
Relevant PR: #544