Well, I tried to upload a PDF for a test and from that, I got this information (in an error message after the upload).
" The following extensions are allowed: .png, .jpg, .jpeg, .jpe, .gif, .avi, .m4v, .mov, .mp4, .ogg, .webm, .wmv, .wav, .mp3, .flac "
Pretty much any photo, audio, or video file type, but no documents. I suppose this mitigates OTT's exposure to copyright infringements, somehow, by not allowing folks to post service manuals, etc. Doesn't stop folks from posting links to the files in cloud storage, though, but a link is not stored data. It's just directions to get to it. I'm not sure how the rules change for a street map to a drop point versus getting someone to hold illegal loot till the recipient picks it up. Maybe there's a difference in receiving stolen property [actual data] and receiving instructions on where to steal it. More importantly, the media type restriction prevents uploading documents with code and 'infectious diseases' such as Excel, Word, Powerpoint, etc documents with embedded code that could easily be malicious.
Another limitation is nothing larger then 5 MB, and a total of about 70 MB. I'm guessing at the total because I didn't look at available space before I put photos on. I show a balance of 64.8MB remaining with about 15 photos. So, keep the files as compact as you can if you want to share lots of photos, or plan on managing what you're sharing. Size, resolution, duration (audio and video) and filetype all play a role in file size, for those of you less savvy about data storage. I have no clue if the same restrictions apply to the public albums. Didn't test that (yet). It may be a per user setting.
If you just click the "+Add Media" button on the Media interface page, you will get this context menu:
Sorry I clipped it off, but below "or" at the bottom are the selections for your personal albums. I just did a window capture and didn't notice until after I'd pasted it in here.
From there you can select any public album to upload your media to, or you can select one of your own albums. I did a little more checking and putting your photos in your own albums gives you some access control. If you put them in public albums, you may forfeit any rights to them at all. You can set access limits on personal albums, but not individual files in the albums. So, you'll need to set up albums based on who you want to let see what, if you have any intentions of restricting access. I didn't look at all the access options, but you can set two access options, View, or Add, and then under that, WHO can do that, to a point. I'll leave to others to look more at that, but the general intent is to allow you to post media that you can have rated by viewers/listeners. As
@Magicman pointed out, it also gives you some convenient storage if you find yourself referencing the same photos over and over. Another thing you can store is links via BB code to other media that is not yours, for instance, a video about the turbo-encabulator. That's probably the least space consuming option if you already have cloud storage that you like to use. You're dependent on that link being valid for a long time, though. As for how this might work for embedded links to photos in your albums, well, I simply don't know (yet). Deleting a photo from an album will quite possibly cause some broken links in message threads. This might be exacerbated by others using links to your photos if you allow everyone access to photos you may not be leaving on the site.
Just remember, ANYTHING you post on here becomes the property of OTT (thus the logo stamped on it), which I'm sure is due to some legal implication of them sharing or forwarding private property/data. Copyright thinks "ye are crunchy and taste good with ketchup" so be careful what you post. If you don't want everyone and his cat to have it, protect it with available settings. If you need to do that, then strongly consider another venue for exchanging the media with whom you want to receive it. Just my opinion on the subject, not trying to stir up any trolls.
Hope this info helps.