Honor overflow=truncate when sending attachments

[pomeloy]

Description:

Related issue (if applicable): #1091

Thanks again for your help @caronc!

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@<this.branch-name>

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  <apprise url related to ticket>

[caronc]

@pomeloy merged! :rocket:

Thank you very much for the PR!