bugfix/dart-dio/multipart-file-handling

[vasilich6107]

in case if we have

      responses:
        '200':
          description: File downloaded successfully
          content:
            application/octet-stream:
              schema:
                type: string
                format: binary

The code fails with Exception

 final rawData = _response.data;
      _responseData = rawData == null
          ? null
          : deserialize<MultipartFile, MultipartFile>(rawData, 'MultipartFile',
              growable: true);

cause deserialize does not have a case for MultipartFile

This PR added handling the case 'MultipartFile':

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[vasilich6107]

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[ahmednfwela]

Question is, should a MultipartFile be returned when response type is application/octet-stream ?
I think it's better to change the return type entirely to Uint8List but I am not sure how

[vasilich6107]

Hey @ahmednfwela
For now it is just a bugfix with deserializtion

[ahmednfwela]

well, this fix will still not fix the bug, since rawData will never be a MultipartFile