mime: use percent-escaping for multipart form field and file names

Until now, form field and file names where escaped using the
backslash-escaping algorithm defined for multipart mails. This commit
replaces this with the percent-escaping method for URLs.

As this may introduce incompatibilities with server-side applications, a
new libcurl option CURLOPT_MIME_OPTIONS with bitmask
CURLMIMEOPT_FORMESCAPE is introduced to revert to legacy use of
backslash-escaping. This is controlled by new cli tool option
--form-escape.

New tests and documentation are provided for this feature.

Reported by: Ryan Sleevi
Fixes #7789
Closes #7805

src/tool_cfgable.h

@@ -239,6 +239,7 @@ struct OperationConfig {
   char *ftp_account;        /* for ACCT */
   char *ftp_alternative_to_user;  /* send command if USER/PASS fails */
   int ftp_filemethod;
+  long mime_options;        /* Mime option flags. */
   long tftp_blksize;        /* TFTP BLKSIZE option */
   bool tftp_no_options;     /* do not send TFTP options requests */
   bool ignorecl;            /* --ignore-content-length */

src/tool_getparam.c

@@ -138,6 +138,7 @@ static const struct LongShort aliases[]= {
   {"$h", "retry-delay",              ARG_STRING},
   {"$i", "retry-max-time",           ARG_STRING},
   {"$k", "proxy-negotiate",          ARG_BOOL},
+  {"$l", "form-escape",              ARG_BOOL},
   {"$m", "ftp-account",              ARG_STRING},
   {"$n", "proxy-anyauth",            ARG_BOOL},
   {"$o", "trace-time",               ARG_BOOL},
@@ -988,6 +989,12 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
           return PARAM_LIBCURL_DOESNT_SUPPORT;
         break;
 
+      case 'l': /* --form-escape */
+        config->mime_options &= ~CURLMIMEOPT_FORMESCAPE;
+        if(toggle)
+          config->mime_options |= CURLMIMEOPT_FORMESCAPE;
+        break;
+
       case 'm': /* --ftp-account */
         GetStr(&config->ftp_account, nextarg);
         break;

src/tool_listhelp.c

@@ -193,6 +193,9 @@ const struct helptxt helptext[] = {
   {"-F, --form <name=content>",
    "Specify multipart MIME data",
    CURLHELP_HTTP | CURLHELP_UPLOAD},
+  {"    --form-escape",
+   "Escape multipart form field/file names using backslash",
+   CURLHELP_HTTP | CURLHELP_POST},
   {"    --form-string <name=string>",
    "Specify multipart MIME data",
    CURLHELP_HTTP | CURLHELP_UPLOAD},

src/tool_operate.c

@@ -1325,6 +1325,10 @@ static CURLcode single_transfer(struct GlobalConfig *global,
         if(result)
           break;
 
+        /* new in libcurl 7.81.0 */
+        if(config->mime_options)
+          my_setopt(curl, CURLOPT_MIME_OPTIONS, config->mime_options);
+
         /* new in libcurl 7.10.6 (default is Basic) */
         if(config->authtype)
           my_setopt_bitmask(curl, CURLOPT_HTTPAUTH, (long)config->authtype);