Add content_type attribute that takes a MIME type to specify the file contents for meshes, hfields, and textures.

PiperOrigin-RevId: 549302344
Change-Id: I30a8adfef5b5d87bb20479329f3396892f5137ff

src/user/user_objects.h

@@ -173,6 +173,10 @@ class mjCBase {
   // load resource if found (fallback to OS filesystem)
   mjResource* LoadResource(std::string filename, int provider);
 
+  // Get and sanitize content type from raw_text if not empty, otherwise parse
+  // content type from resource_name; throw on failure
+  std::string GetAssetContentType(std::string_view resource_name, std::string_view raw_text);
+
   std::string name;               // object name
   std::string classname;          // defaults class name
   int id;                         // object id
@@ -529,6 +533,7 @@ class mjCMesh: public mjCBase {
   // returns a bounding volume given a face
   mjCBoundingVolume GetBoundingVolume(int faceid);
 
+  std::string content_type;           // content type of file
   std::string file;                   // mesh file
   double refpos[3];                   // reference position (translate)
   double refquat[4];                  // reference orientation (rotate)
@@ -652,6 +657,7 @@ class mjCHField : public mjCBase {
   friend class mjXWriter;
 
  public:
+  std::string content_type;       // content type of file
   std::string file;               // file: (nrow, ncol, [elevation data])
   double size[4];                 // hfield size (ignore referencing geom size)
   int nrow;                       // number of rows
@@ -691,6 +697,7 @@ class mjCTexture : public mjCBase {
   int width;                      // width in pixels
 
   // method 2: single file
+  std::string content_type;       // content type of file
   std::string file;               // png file to load; use for all sides of cube
   int gridsize[2];                // size of grid for composite file; (1,1)-repeat
   char gridlayout[13];            // row-major: L,R,F,B,U,D for faces; . for unused