This file is indexed.

/usr/share/sensor_msgs/msg/MagneticField.msg is in libsensor-msgs-dev 1.12.3-5.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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 # Measurement of the Magnetic Field vector at a specific location.

 # If the covariance of the measurement is known, it should be filled in
 # (if all you know is the variance of each measurement, e.g. from the datasheet,
 #just put those along the diagonal)
 # A covariance matrix of all zeros will be interpreted as "covariance unknown",
 # and to use the data a covariance will have to be assumed or gotten from some
 # other source


 Header header                        # timestamp is the time the
                                      # field was measured
                                      # frame_id is the location and orientation
                                      # of the field measurement

 geometry_msgs/Vector3 magnetic_field # x, y, and z components of the
                                      # field vector in Tesla
                                      # If your sensor does not output 3 axes,
                                      # put NaNs in the components not reported.

 float64[9] magnetic_field_covariance # Row major about x, y, z axes
                                      # 0 is interpreted as variance unknown