Add Support for Custom Extension in reassemble_file()

[hbisneto]

The reassemble_file() function currently uses the fixed .fsp extension when searching for split file parts.
To improve flexibility and allow developers to define their own part extensions, a new parameter should be added to the function.

Current Behavior

while os.path.exists(f'{large_file}.fsp{str(i)}'):
    parts.append(f'{large_file}.fsp{str(i)}')

The .fsp extension is hardcoded, which limits the ability to use other naming schemes.

Expected Behavior

Add an optional extension parameter to the function:

def reassemble_file(large_file, new_file, extension=".fsp"):

This parameter allows custom extensions to be specified, e.g.:

reassemble_file("large_file", "new_file", extension=".part")

Acceptance Criteria

• Add extension parameter to reassemble_file() with a default value of .fsp.
• Replace hardcoded .fsp usage with the new parameter.
• Update the function docstring to document the new parameter.
• Keep backward compatibility with existing code.
• Add examples showing usage with custom extensions.

Example

# Default behavior
reassemble_file("archive", "archive_restored")

# Custom extension
reassemble_file("archive", "archive_restored", extension=".chunk")

Additional Context

This change improves flexibility for users who rely on different file-naming conventions during the split/reassemble process.