File replace no longer preserves original filename

Breaking this out from another issue. Original comment by @haleagar at #2271229-18: Allow file replacement extensions for the same file type:

Just a note: this causes a drastic change in behaviour compared in file_entity 7.x-2.0-beta1 and file_entity 7.x-2.0-beta2 vs file_entity 7.x-2.0-alpha.

In file_entity 7.x-2.0-alpha and previous versions if you use the file replace option the original file name (in the file system) was retained and all hard links to that file remain valid.
After this change the file name (in the file system) is changed and any links to the file such as use in a wysiwyg field are broken.

I realize that either behaviour could be argued as correct, but in my opinion the previous behaviour is a more desirable and expected behaviour than the new.
I would say it is *much less* often needed to upload a new image file that has a different extension, than it is to replace a file with one of the same type.

I would suggest that it would be desirable to modify this function such that the file name is only changed when needed by a change in the file extension, or possibly only when expressly requested by the user by a option on the edit form.

We've trained our users to use the file replace tool to update files without breaking URLs all over the place, so for us this is a major regression.

Attached is @haleagar's patch from the other issue.