Unify generation of unique identifier for resumable files

[Floriszenz]

The function generateUniqueIdentifier in resumableHelper.ts generates the IDs differently based on whether files are added directly or as a directory.

For example, selecting all files in a directory and dragging them to a drop area for file selection generates other unique identifiers than selecting the directory itself and dragging it to the area. This is unwanted, both actions should result in the same files / identifiers being added to the file selection.

Therefore, the unique identifier should be generated based on the file name only and not the relative path.

[adi64]

Fixed in #32