support your local gifmaker (#33)

Reviewed-on: #33
Reviewed-by: archvillainette <vickydotbat@tutamail.com>
Co-authored-by: Michał Piasecki <mpiasecki720@protonmail.com>
Co-committed-by: Michał Piasecki <mpiasecki720@protonmail.com>

internal/erf/erf.go

@@ -123,6 +123,7 @@ var extensionTypes = map[string]uint16{
 	"mtr": 0x0818,
 	"jpg": 0x081C,
 	"lod": 0x081E,
+	"gif": 0x081F,
 	"png": 0x0820,
 	"lyt": 0x0BB8,
 	"vis": 0x0BB9,
@@ -188,6 +189,7 @@ var typeExtensions = map[uint16]string{
 	0x0818: "mtr",
 	0x081C: "jpg",
 	0x081E: "lod",
+	0x081F: "gif",
 	0x0820: "png",
 	0x0BB8: "lyt",
 	0x0BB9: "vis",