Wrong image title

Hello, i have a strange phenomenon… 
If i upload my picture (10001.jpg), then it shows in the backend as jpg.10001 and this leads to not being found, if i want to assign it to a product…  image

My REST Resource is like that: 

api/v1/_action/media/d36e1b73af984b8eb6ce62c82ac9dda8/upload?extension=jpg&fileName=10001

Seems to be correct… where is my mistake ?

EDIT:
I tried with renaming the image to “test10001.jpg” and now everything is ok… 
So it seems this function has a problem with a filename, if only numbers are in the filename… 
Any idea for a workaround ?

best regards

Holger