Adds toasts for cases when the file type is not allowed or the file is
too big.
Resolves
https://github.com/orgs/tldraw/projects/53?pane=issue&itemId=70298205
### Change type
- [ ] `bugfix`
- [x] `improvement`
- [ ] `feature`
- [ ] `api`
- [ ] `other`
### Test plan
1. Upload a file that is either too big (over 10mb) or of incorrect file
type (pdf, docx,...).
2. You should see a toast explaining what went wrong.
### Release notes
- Show a toast when uploading an unsupported file type or a file that is
too large (more than 10mb).
adbb0e9b3b