Output a pretty box

J, 25 bytes

echo'. + '{~4|>./~2#|i:12

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Explanation

echo'. + '{~4|>./~2#|i:12
                     i:12  Range from -12 to 12.
                    |      Take absolute values,
                  2#       duplicate every element,
                /~         compute "multiplication table"
              >.           using maximum,
            4|             take mod 4 of every element,
    '. + '{~               index into this string,
echo                       print char matrix for everyone to see.

C, 115 bytes

#define M(x,y)x<(y)?x:y
f(i){for(i=2549;i;i--)putchar(i%51?". + "[(M(i%51-1,M(50-i%51,M(i/51,49-i/51))))/2%4]:10);}

Defines a function f (call as f();) that prints the string to STDOUT.

Pyke, 20 17 bytes

k25V". + "[email protected]

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k                 - out = ""
 25V              - repeat 25 times:
          oh      -     (o++)+1
            e     -    ^//2
    ". + "   @    -   " + ."[^] (wraps around)
              A.X -  out = surround(out, ^)

The surround function was made for kolmogorov-complexity questions like this!