change file permission to 644

[Enoooch]

Issues:
#11
Alamofire:
Alamofire/Alamofire#2527

File system permissions set to -rw-r--r-- which can access images directly, no need to chmod manually.

[kyleduo]

I think we can record the origin permissions and set it back to the new file. Using 644 all the time can be arbitrary.

[Enoooch]

I think we can record the origin permissions and set it back to the new file. Using 644 all the time can be arbitrary.

Yes, setting origin permissions is the best solution.