feat(linux): infer file type from filename.

Author: tai2

lib/linux/index.js

@@ -143,9 +143,11 @@ function linuxSnapshot (options = {}) {
         encoding: 'buffer',
         maxBuffer: maxBuffer(screens)
       }
+      const extension = path.extname(filename) || '.jpeg'
+      const filetype = extension.substr(1)
 
       exec(
-        `import -silent -window root -crop ${screen.crop} -screen jpeg:${filename} `,
+        `import -silent -window root -crop ${screen.crop} -screen ${filetype}:${filename} `,
         execOptions,
         (err, stdout) => {
           if (err) {