### What problem does this PR solve?
Unexpected operation of document management.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Move base64 related function to api/common/base64.py
### Type of change
- [x] Refactoring
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Signed-off-by: jinhai <haijin.chn@gmail.com>
Signed-off-by: Jin Hai <haijin.chn@gmail.com>