Sometimes it's impossible to get a file extension from URL
(eg. http://example.com/gsm/your) so we have to rely on content-type header.
Currenly, asterisk does not support content-type for gsm format(unlike wav).
Added audio/gsm according to https://www.rfc-editor.org/rfc/rfc4856.html
(cherry picked from commit
dab6ddc3227bf5b895dfcc3cda05f81229ddac0c)
static struct ast_format_def gsm_f = {
.name = "gsm",
.exts = "gsm",
+ .mime_types = "audio/gsm",
.write = gsm_write,
.seek = gsm_seek,
.trunc = gsm_trunc,