Change the mail subject when sharing a file to something more consise

[ducalex]

trafficstars

This is related to #28.

I went for the simplest possible, and while testing I found that a prefix was more intuitive than a suffix. We could even remove the prefix IMHO, the filename seems to be enough.

But I'd be happy to change the format to whatever is preferred, as were suggested before!

Examples of the new subjects:

Single file: File: MY_FILE.png Two files: Files: MY_FILE.png and MY_FILE2.png More files: Files: MY_FILE.png and 3 more

[ducalex]

Thinking about it we could even ditch the two files format and only keep: Single file: File: MY_FILE.png 2 or more files: Files: MY_FILE.png and 1 more