uploads: fix getting dimensions of flash files

MediaFile#dimensions is called twice - in #width and in #height but
it only works on the first call because the file is read to the end and
consumed the first time so when #read is called the second time it only
returns the empty string

app/logical/media_file/flash.rb

@@ -5,7 +5,7 @@ class MediaFile::Flash < MediaFile
   def dimensions
     # Read the entire stream into memory because the
     # dimensions aren't stored in a standard location
-    contents = file.read.force_encoding("ASCII-8BIT")
+    contents = File.read(file.path, binmode: true).force_encoding("ASCII-8BIT")
 
     # Our 'signature' is the first 3 bytes
     # Either FWS or CWS.  CWS indicates compression