Send JSON body with SIM900 board I'm trying to send JSON, but i'm not able to set Content-Type header.
Other commands are successful:
[23:49:41:856] AT+SAPBR=3,1,&quot;CONTYPE&quot;,&quot;GPRS&quot;␍␊
[23:49:41:899] ␍␊
[23:49:41:899] OK␍␊
[23:49:52:327] AT+SAPBR=3,1,&quot;APN&quot;,&quot;internet&quot;␍␊
[23:49:52:367] ␍␊
[23:49:52:367] OK␍␊
[23:49:59:585] AT+CSQ␍␊
[23:49:59:594] ␍␊
[23:49:59:610] +CSQ: 18,0␍␊
[23:49:59:610] ␍␊
[23:49:59:621] OK␍␊
[23:50:07:449] AT+SAPBR=1,1␍␊
[23:50:08:501] ␍␊
[23:50:08:512] OK␍␊
[23:50:13:900] AT+SAPBR=2,1␍␊
[23:50:13:928] ␍␊
[23:50:13:928] +SAPBR: 1,1,&quot;127.0.0.1&quot;␍␊
[23:50:13:955] ␍␊
[23:50:13:955] OK␍␊
[23:50:21:261] AT+HTTPINIT␍␊
[23:50:21:291] ␍␊
[23:50:21:291] OK␍␊
[23:50:40:284] AT+HTTPPARA=&quot;CID&quot;,1␍␊
[23:50:40:320] ␍␊
[23:50:40:320] OK␍␊
[23:50:47:652] AT+HTTPPARA=&quot;URL&quot;,&quot;http://127.0.0.1:5000/data&quot;␍␊
[23:50:47:713] ␍␊
[23:50:47:713] OK␍␊
[23:50:54:359] AT+HTTPPARA=&quot;CONTENT&quot;,&quot;application/json&quot;␍␊
[23:50:54:414] ␍␊
[23:50:54:414] +CME ERROR: operation not allowed␍␊
[23:51:06:576] AT+HTTPDATA=38,10000␍␊
[23:51:06:612] ␍␊
[23:51:06:612] DOWNLOAD␍␊
[23:51:12:034] ␍␊
[23:51:12:050] OK␍␊
[23:51:19:386] AT+HTTPACTION=1␍␊
[23:51:19:414] ␍␊
[23:51:19:414] OK␍␊
[23:51:27:747] ␍␊
[23:51:27:758] +HTTPACTION:1,500,290␍␊
[23:51:35:599] AT+HTTPREAD␍␊
[23:51:35:628] ␍␊
[23:51:35:628] +HTTPREAD:290␍␊
[23:51:35:635] &lt;!DOCTYPE HTML PUBLIC &quot;-//W3C//DTD HTML 3.2 Final//EN&quot;&gt;␊
[23:51:35:692] &lt;title&gt;500 Internal Server Error&lt;/title&gt;␊
[23:51:35:735] &lt;h1&gt;Internal Server Error&lt;/h1&gt;␊
[23:51:35:766] &lt;p&gt;The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.&lt;/p&gt;␊
[23:51:35:935] ␍␊
[23:51:35:935] OK␍␊

ip addresses aren't real.
My server received invalid request (with JSON body, but without special header), and replied 500 Internal Server Error.
Revision: 1137B08SIM900M64_ST_DTMF_JD_MMS
Thanks for @Majenko.
I solved problem by upgrade firmware to 1137B15SIM900M64_ST.