Resolve "preprocess is awfully slow"

Bug

Closes #45 (closed)
File.set_content now cause a noticeable delay with the latest GH release

Changes

The process method now writes the lines to an in-memory list and set the content only at the end of said process

Checks

I have verified the following:

• Fixes the issue
• Does not introduces an obvious bug or vulnerability
• Code has been duely tested
• Follows the coding style