Make allowed file endings configurable

[PhilippMatthes]

Fixes: #205

[martinmo]

Thanks @PhilippMatthes, this is a good starting point. Please address my comments and also provide unit tests. Then it should be good to go.

Martin's suggestions were taken into account and added to this branch.

[martinmo]

Looks better, still some points to address. Also, we need to think about case-sensitive vs. case-insensitive. Neither the constance help text nor the test cases mention (or test, respectively) case insensitivity.

[PhilippMatthes]

@martinmo I addressed your recommendations and added another test. Please look into it again. Thanks!

This review is outdated.

[martinmo]

I've added comments of the third review round. There are still some very minor issues.

Concerning the term "file ending", I would prefer to use "filename extension" instead. File ending is not accurate and could also refer to the last character of the contents of a file. Wikipedia also refers to that name (https://en.wikipedia.org/wiki/Filename_extension).

Suggestions were taken into account.

[martinmo]

The validation fails with an AttributeError if I use the manual upload. See comments for the reason. I suggest to adapt the validation function to take a list of filenames as an argument. Use upload.keys() when it is a dict. Use a list comprehension when it is a list of objects.

Update: in the second case, it's of course not just a list of filenames, but a list of file objects instead.

[PhilippMatthes]

@martinmo you are right. Didn't think about that one.

[PhilippMatthes]

I'll fix this tomorrow morning. Sorry for the inconveniences.

Suggestions were taken into account.