mime: fix use of fseek()

Avoid the possible 64-bit offset truncation when used on systems with
small 'long', like Windows.

bonus: make mime_open_file() return bool

Pointed out by ZeroPath
Closes #19100

lib/formdata.c

@@ -771,20 +771,6 @@ static CURLcode setname(curl_mimepart *part, const char *name, size_t len)
   return res;
 }
 
-/* wrap call to fseeko so it matches the calling convention of callback */
-static int fseeko_wrapper(void *stream, curl_off_t offset, int whence)
-{
-#if defined(_WIN32) && defined(USE_WIN32_LARGE_FILES)
-  return _fseeki64(stream, (__int64)offset, whence);
-#elif defined(HAVE_FSEEKO) && defined(HAVE_DECL_FSEEKO)
-  return fseeko(stream, (off_t)offset, whence);
-#else
-  if(offset > LONG_MAX)
-    return -1;
-  return fseek(stream, (long)offset, whence);
-#endif
-}
-
 /*
  * Curl_getformdata() converts a linked list of "meta data" into a mime
  * structure. The input list is in 'post', while the output is stored in
@@ -874,7 +860,7 @@ CURLcode Curl_getformdata(CURL *data,
 #endif
             result = curl_mime_data_cb(part, (curl_off_t) -1,
                                        (curl_read_callback) fread,
-                                       fseeko_wrapper,
+                                       Curl_fseeko,
                                        NULL, (void *) stdin);
 #if defined(__clang__) && __clang_major__ >= 16
 #pragma clang diagnostic pop