Issue explained, see below.

Bug overview description. You cannot exclude more than 1 file type in your search (for example "animated -type:webm -type:swf"). Attempting to do so will only exclude the last file type given.
What part(s) of the site page(s) are affected? Search
What is the expected behavior? In the example, that neither webm files nor swf files are included in the search result.
What actual behavior is given instead? Only swf files are excluded. Attempting to search for "animated -type:swf -type:webm" will include swf files but exclude webm files.
Time of incident (if applicable). N/A
Can you reproduce the bug every time? Yes
What steps did you take to replicate this bug? Attempt to exclude more than 1 file type from any search.
Errors or other messages returned (if any). No error messages are displayed.

Question: why do you need to exclude filetypes when you can just search with the filetype you are trying to seek?

We only support WebM, GIF, Flash and APNG right now, APNG having it's dedicated tag as you can see only has 47 of posts right now: animated PNG

So example animated -type:webm -type:swf just use animated type:gif instead and you get identical results.

Also flash tag should contain all everything under type:swf for blacklisting purposes.

"animated" was just the first word that came up when I wanted to make an example. Arguably the worst example I could've used.

I'm trying to download all image files in my favorites, including gifs, which is why I want to exclude webm and swf files.

Fx2O3 said:
"animated" was just the first word that came up when I wanted to make an example. Arguably the worst example I could've used.

I'm trying to download all image files in my favorites, including gifs, which is why I want to exclude webm and swf files.

Only single one of metatag can be in the search. With animated stuff you can use earlier mentioned tag to get rid of both of those:
animated -flash -type:webm

Mairo said:
Only single one of metatag can be in the search.

That would explain this issue, but I think that should be mentioned in the documentation.