Store names of content files

[bouncepaw]

Currently content files are named like that: <n>.txt and <n>.bin. It was decided that it's not a good naming scheme.

If a revision uses the same binary data as a previous one, it is marked in struct Revision, but not in meta.json. It introduces this bug: if you change a hypha with binary data, the old picture is shown only until you reload the program.

What to do

• Save files with numbers in their stem, but with extension matching their mime-type.
• If it is hard to decide what extension to use, fall back to .txt and .bin.
• Save filenames to meta.json

[bouncepaw]

This commit implemented the new naming scheme but it seems that not many mime-types are provided by stdlib's mime pkg.

TODO: support more mime-types.

[bouncepaw]

After I started transition to 0.7, the problem was resolved. It had been resolved even long before that tho.