fix(linux): Use lookup table on determining filetypes

It makes possitle to derive 'jpeg' from '.jpg'.

Author: tai2

lib/linux/index.js

@@ -133,6 +133,18 @@ function maxBuffer (screens) {
   return total
 }
 
+function guessFiletype (filename) {
+  switch (path.extname(filename)) {
+    case '.jpg':
+    case '.jpeg':
+      return 'jpeg'
+    case '.png':
+      return 'png'
+  }
+
+  return 'jpeg'
+}
+
 function linuxSnapshot (options = {}) {
   return new Promise((resolve, reject) => {
     listDisplays().then((screens) => {
@@ -143,8 +155,7 @@ function linuxSnapshot (options = {}) {
         encoding: 'buffer',
         maxBuffer: maxBuffer(screens)
       }
-      const extension = path.extname(filename) || '.jpeg'
-      const filetype = extension.substr(1)
+      const filetype = guessFiletype(filename)
 
       exec(
         `import -silent -window root -crop ${screen.crop} -screen ${filetype}:${filename} `,