Use bytes for file field on attachment

[ImUrX]

The idea of using the Bytes struct comes from being able to have more flexibility when adding attachments.
Quoting the docs it mentions that

Bytes values facilitate zero-copy network programming by allowing multiple Bytes objects to point to the same underlying memory. This is managed by using a reference count to track when the memory is no longer needed and can be freed.

I would honestly just add an impl Into<Bytes> for the from_bytes function but it would stop being const so it would also require a breaking change.

[7596ff]

Ideally the struct would take &'a [u8] but at the time there were too many lifetime issues that I could not sort out. I went with Vec because I needed to release the change, and it was a temporary fix that ended up lasting longer than it should have. Since Bytes is still the same amount of allocations when using borrowed data, I don't see this change being necessary at the moment.

[7596ff]

Since this isn't a change we are going to be making soon, I'm going to give this a close.