Multipart should use MapFS and MapFile from testing package in standard lib instead of OS

[tjad]

Is your feature request related to a problem? Please describe.
This is an improvement request
Multipart should use MapFS and MapFile from testing package instead of OS. This would be more inline with testing as the file would be in memory and not be required to exist on disk

Describe the solution you'd like
Use https://pkg.go.dev/testing/fstest#MapFS
And https://pkg.go.dev/testing/fstest#MapFile

Describe alternatives you've considered
None - using the current implementation of Multipart in #118 #119 Is almost good enough, but this improvement would make it more complete - we don't want to use the file system during testing. The test data is binary, and storing it in git is not a particularly good way to go. Storing the bytes as integers in a file and writing to disk to be read is inefficient, messy and unnecessary, it is also not a good way to go as it would require cleanup etc etc etc.

[steinfletcher]

Would be happy to take a PR for this, sounds like a good idea.