curl -V
curl 7.29.0 (mips-openwrt-linux-gnu) libcurl/7.29.0 OpenSSL/1.0.1h zlib/1.2.7
Protocols: file ftp ftps http https imap imaps pop3 pop3s rtsp smtp smtps tftp
Features: IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP

curl 7.29.0 (mips-openwrt-linux-gnu)  has limit at string size we pass it and does not report error if over limit.

The work around:

curl -k  -X PATCH  -d @/mnt/sda1/test.base64 'https://shms.firebaseio.com/images/image.json' > /dev/null 2>&1

Put long string at file "/mnt/sda1/test.base64"

Hide standard and error outputs.