Steps to reproduce on regtest:

```
converttopsbt 02000000000101ab73dba46508bacdb49a988ddd74bc9d39c9cb7e057af060ca8c6b6861e28e3c0300000000feffffff02b828d0b20000000017a914d1954862afd728fbb055149002958e1d1b9e1f0b8700ca9a3b0000000017a91442170ccb3441cff1723af9afab6f2553103e1d578702473044022077380c6e16496d9905aef120694c329be51522847cb5d53fa74b94698c95678d02202d32c46e0e17420a83dee830fefda7d40529182f5d82be47c2b49a7b83214ab50121026d483d6628dcc3ef6e3ab9ed4937c452c263c7e0ac80eb4be97426d5da8f640500000000
```
or 
```
converttopsbt 02000000000101ab73dba46508bacdb49a988ddd74bc9d39c9cb7e057af060ca8c6b6861e28e3c0300000000feffffff02b828d0b20000000017a914d1954862afd728fbb055149002958e1d1b9e1f0b8700ca9a3b0000000017a91442170ccb3441cff1723af9afab6f2553103e1d578702473044022077380c6e16496d9905aef120694c329be51522847cb5d53fa74b94698c95678d02202d32c46e0e17420a83dee830fefda7d40529182f5d82be47c2b49a7b83214ab50121026d483d6628dcc3ef6e3ab9ed4937c452c263c7e0ac80eb4be97426d5da8f640500000000 false
```

Further reading: #13738
"      \"unknown\" : {                (json object) The unknown global fields\n"
            "        \"key\" : \"value\"            (key-value pair) An unknown key-value pair\n"
            "         ...\n"
            "      },\n"
            "    }\n"
            "    ,...\n"
            "  ]\n"
            "  \"fee\" : fee                      (numeric, optional) The transaction fee paid if all UTXOs slots in the PSBT have been filled.\n"
            "}\n"
                },
                RPCExamples{
                    HelpExampleCli("decodepsbt", "\"psbt\"")
                },
            }.ToString());
    RPCTypeCheck(request.params, {UniValue::VSTR});
    
    PartiallySignedTransaction psbtx;
    std::string error;
    if (!DecodeBase64PSBT(psbtx, request.params[0].get_str(), error)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
    }
    UniValue result(UniValue::VOBJ);
    
    UniValue tx_univ(UniValue::VOBJ);
    TxToUniv(CTransaction(*psbtx.tx), uint256(), tx_univ, false);
    result.pushKV("tx", tx_univ);
    
    UniValue unknowns(UniValue::VOBJ);
    for (auto entry : psbtx.unknown) {
        unknowns.pushKV(HexStr(entry.first), HexStr(entry.second));