Refactored base64 attachment handling

[caronc]

Description:

Related issue (if applicable): n/a

Refactored the way base64 attachments are generated. removed duplicated code and provided a consistent solution for all attachment types.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 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@base64-encoding-attachment-refactoring

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  syntax://credentials --attach=/path/to/attachment

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.35%. Comparing base (ca50cb7) to head (cda1bd9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1191   +/-   ##
=======================================
  Coverage   99.35%   99.35%           
=======================================
  Files         146      146           
  Lines       20213    20219    +6     
  Branches     3972     3966    -6     
=======================================
+ Hits        20082    20088    +6     
  Misses        121      121           
  Partials       10       10           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.