**Short description of error**
When a .blend file has images (and I assume other external assets as well) with absolute paths, bam pack throws an `AssertionError`. When there is an asset that has an absolute path, it should do one of two things:
# Convert those paths to relative paths and pack normally
# Keep the paths as absolute paths in the newly packed files (but don't throw an `AssertionError`)
In my situation, I need the first option. But the second may be useful in a different scenario so maybe there should be an option to choose which method it should use for handling absolute paths.
**Exact steps for others to reproduce the error**
Try to pack the attached file (`python -m bam.pack --input <file path> --output <output file path> --mode FILE`), because there is an image in the file with an absolute path, it will fail with an `AssertionError`.